# What is the right pressure



## wp1234

Sorry to bombard the forum with Autotrail questions but I’m a tad confused having had my Cheyenne 696G Fiat serviced last week .The garage mechanic told me that I was running with tyres underinflated and he had topped them up to 79PSI ( as per door pillar sticker )
However looking the tyre wall info (Continental Vanco camper 225 75R 16CP ) it seems that the max PSI should be 69 and other forum posts seem to suggest that 79 Psi is high for a cold tyre .
The difference between the two possible pressures seems a lot to me so what do other Cheyenne 696G owners inflate their tyres to ?


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## lookback

When I put my autotrail into Camper UK for a service they pumped my tyres upto 80psi (cos that what it says on the sidewall for USA nad Australia!) I normally have 60psi in the front and 65 psi in the rear and find the ride OK.

The only sure way to find out is to go to the public weighbridge (when you are fully loaded) and get each axle weighed then contact the tyre manufacturer for their recomendations.

Ian


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## wp1234

lookback said:


> The only sure way to find out is to go to the public weighbridge (when you are fully loaded) and get each axle weighed then contact the tyre manufacturer for their recomendations.
> 
> Ian


Have mailed Continental tonight to see what they say - Tyre pressures are a bit of a dark art I suspect


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## CliveMott

You should NEVER inflate tyres to a pressure greater than the maximum given on the sidewall of the tyre.
If the vehicle rating plate requires a pressure above the sidewall rating then you have the wrong tyres fitted.

Under-inflated tyres may produce a softer ride but will seriously impare cornering and increase problems associated with side winds.

Double check the figures again perhaps?

C.


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## loughrigg

I had a similar experience with a "helpful" mechanic - but he didn't think it necessary to tell me that my tyres were now inflated well above their design maximum. Suffice to say that it didn't improve handing and I nearly had a close encounter with a ditch.

If that is an indication of the level of training in the workshops at Camper UK, I won't be rushing to book my MH in at their establishment.

Mike


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## wp1234

CliveMott said:


> You should NEVER inflate tyres to a pressure greater than the maximum given on the sidewall of the tyre.
> If the vehicle rating plate requires a pressure above the sidewall rating then you have the wrong tyres fitted.
> 
> Under-inflated tyres may produce a softer ride but will seriously impare cornering and increase problems associated with side winds.
> 
> Double check the figures again perhaps?
> 
> C.


Hi Clive ,
Thanks for the response .
Been out with the torch again, seems my tyre wall has two presure numbers one says 69psi ( is this the USA load pressure perhaps) and the other is 5.5bar (80 psi ) . The sticker on the door pillar says 225 75R 16CP tyres should be inflated to 79.5 psi back and front.

Wyn


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## loughrigg

CliveMott said:


> If the vehicle rating plate requires a pressure above the sidewall rating then you have the wrong tyres fitted.
> C.


If I have tyres fitted that are the correct size, with appropriate weight and speed indices, on what basis would you consider them to be incorrect tyres for the vehicle?

Mike


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## peribro

There have been a number of other posts about this topic and so far as I can ascertain the issue is still currently unresolved. I am running with the same tyres as the OP and have raised the same query with Continental. I am copying below the reply that I have received from Continental but I have gone back to them to query it as their recommended pressures (for the front in particular) are so much less than the 79.5psi shown on the door pillars and also advised by the dealer. I think that the answer will be to do with the relevant loadings but would like it confirmed.

There are five tyres within this size that have differing service
> descriptions - 116/114N(110S) - 116/114R(118/116P) - 116R - 118/116R &
> 121/120R. However, there are two sets of pressure recommendations for your
> particular loads so you need to ascertain what you have fitted to your
> vehicle before adjusting the pressure.
> Please find the pressures, below.
>
>
> For tyres with the service description: 116/114N(110S) - 116/114R(118/116P)
> & 116R
>
> Front:
> 3 bar - 43psi which carries a maximum load of 1730kg
>
> Rear:
> 4.5 bar - 65psi which carries a maximum load of 2395kg
>
>
>
>
> For tyres with the service description: 118/116R & 121/120R
>
> Front:
> 3 bar - 43psi which carries a maximum load of 1685kg
>
> Rear:
> 4.75 bar - 69psi which carries a maximum load of 2435kg
>


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## loughrigg

When I bought my MH I looked for new tyres to replace the badly cracked ones that were then fitted (six year old Michelin Campers).

Deep in the handbook was the comment that the door pillar pressures were those recommended for a specific Michelin tyre that was fitted on the production line (not a camping tye and, in any event, no longer available) and at an assumed level of loading.

It seems logical (to me) that those initial pressure guidelines only continue to apply for as long as the vehicle is fitted with an identical tyre or one with the same load/pressure characteristics.

At some later point, if a tyre is fitted that is correct in terms of size, speed index and weight index but is made by Continental (for example), why would it make sense to inflate that tyre to the pressures recommended by Michelin for one of their old tyres.

If you had a car originally powered by unleaded petrol that you chose to convert to LPG, would you still pump it full of unleaded petrol because that it what it says in the original handbook?

Mike


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## DJP

Have a look at this link here from the Autotrail Owners club website
ATOC TYRE INFO
Go to NEWSFLASH and scroll down to tyre info


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## CliveMott

If the tyre pressures required by the vehicle manufacturer (for stability reasons most likely) are in excess of the maximum pressure permissable for the tyre then the tyres are wrong for that vehicle. The may be OK re weight and speed but not rigidity to stop body roll.

Ask Autotrail formerly and directly I suggest.

C.


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## Wizzo

CliveMott said:


> If the tyre pressures required by the vehicle manufacturer (for stability reasons most likely) are in excess of the maximum pressure permissable for the tyre then the tyres are wrong for that vehicle. The may be OK re weight and speed but not rigidity to stop body roll.
> Ask Autotrail formerly and directly I suggest.
> C.


But the sticker on the door pillar would be attached at the time of manufacture of the cab without knowledge of what is going to be tagged on the back end. The 5.5 bar recommendation is surely the manufacturer (Fiat/Peugeot etc) playing safe so that there is no possibility of under inflation. I cannot believe that these are the optimum pressures in every case.

My van comes with the same door sticker but also with a converter's recommendation of 43psi, a very big discrepancy.

JohnW


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## loughrigg

CliveMott said:


> The may be OK re weight and speed but not rigidity to stop body roll.
> C.


Interesting. That suggests a simple correlation between tyre pressure and tyre rigidity irrespective of manufacturer or construction.

That's not a point I recall being discussed before - is it really that simple?

Mike


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## wp1234

CliveMott said:


> Ask Autotrail formerly and directly I suggest.
> 
> C.


I will do that at the NEC show in the morning .Seems crazy that there is so much confusion about tyre pressures on here .
It also does seem to me a little daft when the normal running pressures on the door pillar are so close to the max allowable marked on the tyre .I

Thanks all


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## Bacchus

Lookback is correct. The correct tyre pressure is 60/65psi. Well, at least that's a good starting point.............


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## jadatis

What is on the sidewall of LT tires is the reference-pressure, and not the maximum pressure of the tyre. When standing still , you may even fill them with 1.4 times as much pressure to bare 2 times as much weight.
I got hold of the formula the tyre- and car- manufacturers in Europe use to determine the advice-pressures for cars, and learned myself Excell to make spreadsheets for it.
http://cid-a526e0eee092e6dc.skydrive.live.com/self.aspx/.Public/pressurecalculationwithtemp.xls
This one is the easyest to use for motorhomes , but from this link you can navigate trough my complete public map of hotmail to find more spreadsheets and articles for tyre-pressure.
So it is not a desaster to fill them above the pressure on the side-wall.
But you could experiënce a bumby ride.
My own input is the Load-percentage.
If you take care that the real weight on the tire is not under 85% of the weight the pressure is calculated for, you have a comfortable ride.
Going over 100 % can damage the tires at higher speeds.
The reason for going over it is that the rear axle often goes over the weight the axle may bare, and so with a lower speed you dont damage the tires then. at 100km/h you may add 5% to the max load of the tire when adding 6 % to the pressure.
But with bigger tires wich can bare much more then the maximum Axle weight ( GAWR) that are nowadays used at newer motorhomes the reference pressure or even lower gives anaugh reserve for overloading.


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## Bacchus

DJP said:


> Have a look at this link here from the Autotrail Owners club website
> ATOC TYRE INFO
> Go to NEWSFLASH and scroll down to tyre info


Picked up my Chieftain last week and Continental Motorhomes gave the tyre pressures as 79.5psi front and rear. I've checked the web page you gave and I'm still confused. We're talking about 2 back axles and common sense would tell me that the pressures on the back should be less than the front as each tyre has less total weight to support. Am I missing something ? - that's what is implied in the link you provided.


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## skiboycey

When I got my 696G the tyres were at 4 bar and it worked OK but the ride was wallowy and the handling not so sharp.

I then tried 4.5 bar which was better and also the maximum pressure most service stations were able to supply so I drove it like that for a couple of months.

I then bought an expensive footpump and after about an hour of sweaty foot pumping (600 pumps per tyre!) I got the tyres to 5.5 bar. I then discovered the electric inflator in the repair kit stowed under the front seat and spent 10 minutes swearing!

At 5.5 bar the handling is certainly better and much tighter than at 4.5 bar. The ride is a bit more bumpy but after 20 minutes you get used to it and don't really notice it anymore.

If you are slow and cruisy then I'd recommend 4.5 bar as it's more comfortable. if you push on a bit harder or drive in mountains a bit then 5.5 bar gives much sharper handling. I suspect (but don't actually know) that the higher pressure will better protect the tyre sidewall when laid up as well.

The tyres had 80 psi marked as maximum and were within load tolerances so I assume Auto Trail won't be risking a huge liability claim should one explode at the recommended pressure and that they have done their homework properly and can be relied on, at least with the original tyres fitted.

Regards, Mark


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## Dukeham

Hi.
I have a Burstner Aviano 4ton chassis. My supplying dealer & the dealer that did the hab/damp check both told me that Fiat recommend 80psi as pressures. They both agreed that this was too much but could not come up with a suitable pressure.
At 80 psi the van is all over the road with the slightest undulation. I will be dropping mine down to 70psi to see what that is like. My tyre size is the same as the O/P.
This subject does seem to be a very controversial one !!!!!!!
GC.


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## peribro

I have contacted Continental Tyres twice on this - the second time to make sure that I had not misunderstood the advice they had given as it was so different to other advice I had seen - in particular the maximums given on the tyre wall and on the Fiat door pillar and in the Fiat handbook. A Product Support Engineer from Continental has reconfirmed that the front tyres on my Autotrail should be inflated to 3.0 bar (43psi) and the rears to 4.75 bar 69(psi). This is based on my axle loadings of 1610kgs and 2350 respectively. I have now dropped the pressures to 50psi and 70psi respectively and will see how it goes.


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## skiboycey

I would think the steering would be very wallowy at 3 bar... Mine was quite bad at 4 bar.

I'd be interested to know how it handles with these low pressures and if the front tyres get very hot in use which often happens at lower pressures.

Cheers, Mark


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## jadatis

peribro said:


> Engineer from Continental has reconfirmed that the front tyres on my Autotrail should be inflated to 3.0 bar (43psi) and the rears to 4.75 bar 69(psi). This is based on my axle loadings of 1610kgs and 2350 respectively. I have now dropped the pressures to 50psi and 70psi respectively and will see how it goes.


Then I have calculated back that you have tyres with a load-index of 114 with 1180 kg per tyre maximum load and reference pressure ( "at 475 kPa "or "at 69psi") of 4.75 bar. But there is a posibilty they calculated it from a higher loadindex but then also higher reference pressure of say 80psi , so you yourselfes can see on the tyre-side-wall if I am right.
And that they based their advice with the formula I also use in my spreadsheets. 
The Gross axle weight rating ( GAWR front) front of 1610 gives a advice pressure of 2.94 bar rounded up to 3.0 bar, and with the 69 psi at the back you can bare 2* 1180= 2360 kg maximum.

My advice would be, because the back- axle load easy goes over its maximum to put some more pressure in it .
That is allowed up to 1,4 times as much then the reference pressure. you can read about that in this german article at page 10 to 12 of semperit. 
http://cid-a526e0eee092e6dc.skydriv...chtdruktabel^_bandendraagkracht^_Semperit.pdf

6% more pressure so 74 psi gives up to 100km/h / 62M/h maximum speed a extra of 5% so 100kg so 2460 kg bearing 
If the axle-weight stays under 2360 kg ( 1180 per tire) you may drive faster , but the weights are mosty under estimated.
And you have to take care of loaddiferences between left and right on the same axle, even if you keep it to 2350 at the back.
Say diference of 80kg gives 1215 kg on one side so over the maximum load of the tyre.

The front can realy be held so low because the front-axle never gets overloaded, or you have to put some heavy lugage on the bumper. So with the pressure of 3 bar you even have a savety factor of about 5% at least, so less warmer tires.
There is a chance of a bumpy ride because of the front tire pressure of 3.5 bar if you have less then 1510 kg on the front axle or 755kg on one tyre of it. But first for your own savety feeling use that , and if you experiënce a bumpy ride, you know you can savely go lower to 3 bar where the front axle weight can go back to 85% of 1610 kg = 1370 kg before the fillings come out of your teeth.
Then you have more grip on the road for difficult situations .


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## jadatis

peribro said:


> There are five tyres within this size that have differing service
> > descriptions - 116/114N(110S) - 116/114R(118/116P) - 116R - 118/116R &
> > 121/120R. However, there are two sets of pressure recommendations for your
> > particular loads so you need to ascertain what you have fitted to your
> > vehicle before adjusting the pressure.
> > Please find the pressures, below.
> >
> >
> > For tyres with the service description: 116/114N(110S) - 116/114R(118/116P)
> > & 116R
> > maximum load 1250kg at 69 psi
> > Front:
> > 3 bar - 43psi which carries a maximum load of 1730kg
> >
> > Rear:
> > 4.5 bar - 65psi which carries a maximum load of 2395kg
> >
> >
> >
> >
> > For tyres with the service description: 118/116R & 121/120R
> > Maximum load 1320 kg at 80psi ( 5,5 bar)
> > Front:
> > 3 bar - 43psi which carries a maximum load of 1685kg
> >
> > Rear:
> > 4.75 bar - 69psi which carries a maximum load of 2435kg
> >


By reading back the topic I saw that you already gave the loadindexes they based their advice on.
Yust look at the tyre-side wall to see wich one you have.
I placed the reference pressure, wich I calculated back, in the quote
in red. 
I asume that they dare not to go lower then 3 bar for LT tyres.
I have my own idea for that , and that is if you go lower then 60% of the reference pressure so 66,6% ( 2/3) of the maximum load, you have to calculate lineair from there.
Then 3.3 bar is 60% of 5,5 bar or 48 psi and so you have to calculate for the second mentioned tyres for the front 1610kg/(2640kg*0,6666) wich gives rounded up 44 psi so 3.1 bar
for the first tyres you can even go as low as 40 psi or 2,8 bar 
Maybe a stiffer tire must have a higher pressure percentage then 60 % of RP.
I based this on the remark at the formula I have for normal car tires where they write that you may not go lower then 1.5 bar and Ref press is 2,5 bar so 60%. Chance is that the tyre comes loose from the rimm and looses pressure, is written, but also the tyre-side-wall can get to warm at higher speed, and get damaged.

If you want to read more about a new sugestion of calculating tyre-pressure and the comparison with the way it is done now in Europe and America, you can read this article in PDF.
http://cid-a526e0eee092e6dc.skydrive.live.com/self.aspx/.Public/tirepressureatreducedloads.pdf
With that kind of calculating you dont need the 60% rule I concluded, if that one is a good one, it was only a suggestion.
What it proves is that the European way is the savest calculation up to now.


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## wp1234

This weekends mission was to find out exactly what my vehicle weighed in at fully loaded and fueled up ready for the road , and then to get onto Continental to determine what the right pressures are for my 225 75R 16CP (116R ) tyres .

From the local VOSA weighbridge :

FRONT = 1700 Kg
REAR = 2240 Kg
TOTAL = 3940 Kg

This back from Continental:

-----Original Message-----
From: [email protected] [mailto:[email protected]] 
Sent: 08 March 2010 10:44

Please find the details for your weights , below.

Front:3 bar - 44psi

Rear: 4.25 bar - 62psi

Regards
Product Support Engineer

How does the panel view the pressures recommended by Conti?


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## Rudderman

*Tyre Pressures*

During the last 6 years I have emailed Michelin three different occasions regarding my XC Camping tyres on my Sundance. Each time, giving them the same maximum axle weights, they have given me three different sets of pressures    
Do the Tyre Manufactures really know?

What pressures would the Police look at on a roadside check?


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## jadatis

Then I would calculate for the back with 2240 x 1,1 for reserve for extra load or load diferences left and right .
So 2462 kg , and come with the same calculation they use to 69 psi.
Then the weight can go back to 85% of 2462 kg = 2100kg( or 1050 kg per tire) before discomfort by bouncing. 

Because the front here is weighed you also could add 10% for savety in case of weightshifting R/L and other loading on the way back.
1700*1.1= 1870kg gives 49 psi. then the real weight on the front-axle can go back to 85% of 1870 kg = 1590 kg ( or 795 kg per tire) before discomfort.

I will look back in the topic to see if you mentioned the GAWR's already.


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## wp1234

Thanks for this JADATIS . 
Given your explanation I think im going to set them at Front - 50 Psi ( 3.5 Bar) and Rear - 65 Psi ( 4.5 Bar ). This seems to be the optimum tyre pressures for a loaded 696G taking all the forum comments into account plus tyre manufacturers guidelines.
I just wish someone would tell Fiat Service stations that 80psi isn't right .


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## skiboycey

"I just wish someone would tell Fiat Service stations that 80psi isn't right ."

Respectfully I think you're missing the point. There is no 'right' tyre pressure, just a recommended one which, in the opinion of the manufacturer or as the result of an algorithm, gives the best compromise of desired characteristics.

If you go for lower pressures you'll get a nice ride and, maybe, more tyre in contact with the road if you need it at the expense of extra heat generated and poor handling if you drive a bit faster round bends than average. If you go for higher pressures you'll get nice light steering, better handling and lower temperatures at the expense of a rougher ride and perhaps uneven wear on the centre of the tyre. Broadly speaking I think you'll get better fuel economy with higher pressures as there's less rolling resistance as the tyre becomes more rigid.

There's no right, and no wrong! Having tried all the variations on my Cheyenne I've decided 80psi makes the handling so much better that I'll put up with the bumps coming through when driving. Others will make a different choice...

I do know under inflated is more dangerous than over inflated due to tyre movement on the rim generating heat in the sidewalls so it's usually best to go a bit higher if in doubt. In fact when I was a kart racer we used to run the slicks at 8 psi in cold weather to get some heat into them so they would grip but 18psi in the middle of summer.

Cheers, Mark


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## Rapide561

*Tyre*

Different motorhome, a Kontiki tag axle, but I run at 80 psi all round.

Russell


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## zulurita

On my Cheyenne 660 which is 4000kg I tend to do 75 or 76 psi I found 80 psi a bit too hard and I don't like pumping up to max allowed.

I tend to do pressures same at front and back. Reading posts here it seems some do lower at front. Doesn't seem to be recommended in manual so I am wondering whether I should keep mine the same all round or not?


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## peribro

Kwik-fit has a "picture" on its website of what correctly inflated, under-inflated and over-inflated tyres look like. I will have a look in the daylight tomorrow and see whether mine look anything like the ones in their picture!

http://www.kwik-fit.com/assets/jpg/graphics/tyre-pressures.jpg


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## wp1234

‘There is no 'right' tyre pressure, just a recommended one which, in the opinion of the manufacturer or as the result of an algorithm, gives the best compromise of desired characteristics.’

Respectfully again , no doubt your right it’s for everyone to make their own choice and I’m sticking as close as I can with the tyre manufactures recommendation anything else is a guess , in the event of an accident I don’t fancy my luck in court with that one .

Russian roulette anyone!


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## skiboycey

"n the event of an accident I don’t fancy my luck in court with that one . 

Russian roulette anyone!"


If you never crash you'll never end up in court... 

If you were unfortunate enough to end up in court and the police attempted to say your tyres were the wrong pressure AND this lead to the accident or contributed to the severity of it I'd imagine the contents of these forums indicating no consensus even from the manufacturers to be an excellent defence if your lawyer was any good.

Unless they exploded from being at 120 psi or were so severely under-inflated as to lead to extreme handling problems I'd imagine this would be extraordinarily hard to prove.

like so much of modern life what people are really looking for is for somebody else to make a decision for them and feeling very uncomfortable when the 'correct' answer to their question is that there 's a range of acceptable pressures and it's up to you to pick the particular trade off you are happy with. These same people then complain when Fiat put 80 psi on the door frame because it gives a harsh ride but probably results in the fewest legal problems for Fiat in the event of liability claims as under-inflation is more dangerous than over inflation.

On the one hand you want the decision made for you and on the other hand complain when you don't like it or seek an even higher authority in the shape of a tyre company who are prepared to give the answer you are looking for!

A parody of modern life, indeed... 

Regards, Mark

PS - no matter what the tyre company says I really think 3 bar is too low for the front of a 4000kg X2/50 van. It felt too low at 4 bar to me. I'd get written confirmation first as I really think at that pressure something bad could happen. I say that not to be smug but because I know a fair bit about cars from racing them for years and learning to 'feel' the right pressure of a tyre in testing and that pressure/cornering load combination just doesn't feel right to me. The tyre would get very hot, especially with the engine pulling a big, heavy habitation unit. I'd hate to see an accident happen because I kept my mouth shut but of course it's your decision if you decide to run them that low. Remember that commercial history is littered with examples of poor information from manufacturers. Good luck...


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## wp1234

'If you never crash you'll never end up in court... '

Did I say crash, heaven forbid  

What I said was accident which of course I will do my utmost to avoid :wink: 

I take your point and valued advice on the front, and seemingly low, Conti pressure advice and on reflection I will up that a bit . 

Given that the heavier weight is at the back end I would say that there should at least be some sort of differential between front and rear pressures or is that an erroneous assumption also ? 
Thinking on 65 front 75 ish on the back end will do me


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## skiboycey

When I said 'you' in that last post it was in the general 'you' meaning everybody in the world and not 'you' in particular.

English is occasionally such an imprecise language. This would never happen in French... 

Sorry about that it looked like I was having a go at you...

I'd play around with the pressures. 65 was perfectly good on the Cheyenne but I didn't like the body roll and imprecise steering. However the ride was much nicer than at 80. If it's any help I always run cars about 4 to 8 psi higher than recommended and I've never had a problem in 20 years of driving. In general the only time I've ever found it useful to run lower pressures is in snow when it can give some more grip on wide and fat tyres.

Cheers, Mark


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