# Half A Tank of Fuel. Does it greatly affect fuel economy ?



## Freddiebooks (Aug 9, 2008)

You don't need to be a genius to know that the lighter the load the less fuel you use. This is why we only fill water tanks when we need to and empty them asap before setting off to somewhere else. 

So.... can any of the clever people tell me, does it greatly affect fuel economy to only fill the tank half full or less so basically your not lugging around lets say 50 litres of fuel that you are not going to use for the next 200 or so miles. 

I'm just thinking out loud, a dangerous game as this may well be a well troden subject already.

Any thoughts Anyone ????

Freddiebooks


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## asprn (Feb 10, 2006)

*Re: Half A Tank of Fuel. Does it greatly affect fuel economy*

Travelling with less weight is always going to be cheaper, although just how much will be down to the likes of Frank to work out. :roll:

I learnt something from a guy who was on the next pitch to us at La Manga last March (2007). He said that he arrived with an empty tank, but filled it to the absolute brim at the service station just along the road before pitching up for several months over winter. It apparently stopped water forming on top of the fuel when parked up.

Just a thought if you're diesel and intending to park up for a while.

Dougie.


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## krull (Jul 22, 2006)

*Re: Half A Tank of Fuel. Does it greatly affect fuel economy*



Freddiebooks said:


> This is why we only fill water tanks when we need to and empty them asap before setting off to somewhere else.
> 
> Freddiebooks


I don't bother. Always travel with full water (& empty waste) when I can. Wild camp and use aires so can't guarantee that I can always fill up when I fancy.

Don't think it makes any real difference.


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## chapter (May 1, 2005)

in my case no! 
diesel in bristol is a lot cheaper than north devon or north cornwall it can be as much as 30p a litre 
but france thats a another story 
chapter


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## Carper (Aug 15, 2007)

*Re: Half A Tank of Fuel. Does it greatly affect fuel economy*



Freddiebooks said:


> You don't need to be a genius to know that the lighter the load the less fuel you use. This is why we only fill water tanks when we need to and empty them asap before setting off to somewhere else.
> 
> So.... can any of the clever people tell me, does it greatly affect fuel economy to only fill the tank half full or less so basically your not lugging around lets say 50 litres of fuel that you are not going to use for the next 200 or so miles.
> 
> ...


Hi

I'm not sure how much difference it would make.

It doesn't seem to make any difference whether our tanks, water and diesel, are full or empty. What does make a difference is speed, as the average Motorhome has the aerodynamics of a house brick

Doug


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## Wanderwagon3 (May 27, 2005)

*As Topic*

Freddiebooks...

The savings ,if quantifiable,and as a %age of total trip costs,of running light in fuel and water.do not ,in my opinion, outweigh the serious inconvenience of running out of water or fuel.

We always keep topped up with water( inc'g toilet flush tank) and keep fuel tank to brim after it gets down to a third full.

Also empty the toilet and used water tank at every opportunity.

I don't look for avoidable hassle.

Ken.............with Wanderwagon3


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## Rapide561 (Oct 1, 2005)

*MPG*

Hi

In my opinion, it makes beggar all difference to MPG having a full fuel tank. In fact, I travel with full fresh water tank too. My van is loaded upto about 4850kg vice a 5000kg maximum, and mpg is not affected by anything such as weight or to an extent, speed. At between 58 and 70mph, I get the same mpg on a long run. At 55 ish I get a bit more to the gallon, but I would rather drive my motor and giv it some welly.

The previous van - with Luton highline was the same - weight was not an issue.

The largest factor is the co-efficient drag factor and hence the low line is doing better than the Luton model did.

Towing the car behind me knocked about 7% off the "long term" average.

Russell


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## waz (Jan 5, 2008)

When I used to do removals between UK/Spain with a 3.5 ton Iveco it made no difference in fuel consumption fully loaded or empty. What did make a difference was not using the motorways. On the motorway 6.5ks per litre of the motorways up to 8ks per litre.
Waz


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## safariboy (May 1, 2005)

Presumably this is because you were going faster on the motorway? At the same average speed I would expect the constant motorway speed to be more efficient.

Mass will make a small difference when accelerating or hill climbing but as the previous entries say it will be almost undetectable.


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## gromett (May 9, 2005)

It does make a difference if you are travelling great distances and know exactly where you can fill/empty. The difference you are talking about though is in pennies not pounds. I have travelled through france a number of times and after running out of diesel once, never let it get below 1/2 a tank now. The costs difference is pretty minimal to be honest.

The biggest difference you can make to the cost of fuel is to take a couple of grams of lead out of your right shoe. over a 1200Mile trip it made aprox £50 difference doing 55 mph as opposed to nailing it as much as I could (allowing for speed limits).

The water/diesel tanks made "practically" no difference on similar trips.

Karl


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## aultymer (Jun 20, 2006)

> You don't need to be a genius to know that the lighter the load the less fuel you use. This is why we only fill water tanks when we need to and empty them asap before setting off to somewhere else.


What a waste of water for an imagined fuel saving and the chance of being caught short (literally) if you are in a queue for a delayed ferry or train or even just a traffic jam. I know tuggers do this routinely (look at the patches of dead grass where they dump hot water) but there is no reason to do this in a motorhome. By all means empty the dirty stuff out but I suggest you fill with clean water and fuel whenever possible since you never know how long it may be before your next opportunity to do so (unless you only move between predetermined camp sites).


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## teemyob (Nov 22, 2005)

*Quote*



aultymer said:


> > You don't need to be a genius to know that the lighter the load the less fuel you use. This is why we only fill water tanks when we need to and empty them asap before setting off to somewhere else.
> 
> 
> What a waste of water for an imagined fuel saving and the chance of being caught short (literally) if you are in a queue for a delayed ferry or train or even just a traffic jam. I know tuggers do this routinely (look at the patches of dead grass where they dump hot water) but there is no reason to do this in a motorhome. By all means empty the dirty stuff out but I suggest you fill with clean water and fuel whenever possible since you never know how long it may be before your next opportunity to do so (unless you only move between predetermined camp sites).


Did you read the post correctly. As an example, they did not state they emptied the water hot cold or otherwise over grass!

Trev.


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## aultymer (Jun 20, 2006)

> Did you read the post correctly


Did you read my post correctly? 
I did not at any time mention any motorhome emptying water (hot or cold ) on grass. I merely referred to the waste of water (hot or cold) no matter where it is dumped. 
I did mention the deplorable habit of tuggers emptying hot water on grass as can be seen on many campsites. 
Apart from this erroneous example, what else prompted you to think I had not read the post?


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## teemyob (Nov 22, 2005)

*Aultymer*

Hello Aultymer,

I just did not think the poster meant he emtied the fresh water more the waste.

Maybe we both missed something.

Trev.


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## aultymer (Jun 20, 2006)

Only the OP can confirm if he meant that he dumps perfectly good fresh water "asap" or if it is just his waste he dumps.
Agreed we are both missing something - a need to sleep at this time and more info on the question!!!


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## geraldandannie (Jun 4, 2006)

I remember reading somewhere that the vast majority of fuel consumption, especially at cruising speeds, coems from the aerodynamics of the vehicle, and the resistance caused by trying to push a vehicle through the air.

50 litres of water weighs 50kg, which is something like 1.4% of the weight of a 3.5t motorhome, which would have very little affect on fuel consumption. Diesel fuel has a slightly lower specific gravity, and is therefore lighter than water.

Gerald


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## Boff (May 10, 2005)

Hi,

OK, let's do some science: :wink:

As long as you are driving at constant speed on a level road, additional weight makes practically no difference at all. There is a tiny little increase of friction between tyres and road, but that is not measurable.

The situation changes if you accelerate or go uphill. Then additional energy from the engine has to be brought up to either accelerate the additional weight (kinetic energy), or carry it up the hill (potential energy). In both cases the additional energy will later be converted into heat and therefore be lost. In both cases, the amount of additional energy, and therefore fuel, is proportional to the amount of added weight. So, *while going uphill*, a 5% increase in weight would result in a 5% increase in fuel consumption.

However, even Mt Everest has a summit, and once you are over the summit it is absolutely inevitable that you go downhill again. And when going downhill the additional weight will not help fuel consumption, but it will not harm either. The same is valid for acceleration. So, as a *worst case scenario* we can assume that on 50% of the travelled distance the additional weight will matter.

Now my van has, if loaded to the brim but with empty tanks, a weight of about 3.3 tons. And the fresh water tank can hold 150 litre, so 150 kg of water. So would I fill the fresh water tank before departure, I would add 4.5% of weight. Which means an increase in fuel consumption of max. 2.3% in the worst-case scenario! So, instead of let's say 25MPG it would make 24.4MPG.

Regarding the fuel tank it is even more questionable: Diesel fuel has a density of only about .835 kg/litre. A typical MH fuel tank holds about 85 litres, so would I make it half full I would save 42.5 litres, so about 35.5kg. Which would result in a maximum possible mileage increase of 1%.

Now is this really worthwile?

I believe not, and therefore we always start with fresh water filled up to the brim. And with the fuel we consider the price differences in the countries we intend to travel. Which usually means again that I fill up, because seen from my home country Netherlands all neighbouring countries have more expensive diesel.

Best Regards,
Gerhard

P.S: With passenger cars this is slightly different, as their unladen weight is considerably lower than that of a motorhome.


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## Alemo (Mar 11, 2008)

Hi Gerhard,

Now that is how to answer a question.

Well done.

Alec


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## Boff (May 10, 2005)

Thanks, Alec!

However, I have made a mistake which made my calculation even too optimistic. So my "final verdict" is:

*If you fill your fuel tank only half, then the maximum possible gain in mileage will be below 0.5%!*

'nuff said.

Best Regards,
Gerhard


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## oldenstar (Nov 9, 2006)

I concur with the majority here.
Always set off with a full (130 litres) fresh water tank, and a full tank of diesel, but try to empty the waste tank as frequently as possible (in the proper places I hasten to add).
However last week we did about 700 miles to the NEC, visiting relatives etc before returning home, and my mpg dropped by about 2 mpg to 28.5.
This was down to two things in my opinion-First the strong winds on Saturday and Sunday, but mainly due to my new Syrius Truckmate Satnav.
This gives a very accurate true speed reading which showed that I had actually only been doing about 50mph, so I stepped it up to a dizzying 56mph with the cruise control-That 5 or 6 mph equated to about 2mpg!!.
Incidentally also topped up my lpg cylinders for the first time-13.77 litres at a cost of £8.25.-Economical and easy.

Paul


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## AberdeenAngus (Jul 26, 2008)

*Re: Half A Tank of Fuel. Does it greatly affect fuel economy*



asprn said:


> I learnt something from a guy who was on the next pitch to us at La Manga last March (2007). He said that he arrived with an empty tank, but filled it to the absolute brim at the service station just along the road before pitching up for several months over winter. It apparently stopped water forming on top of the fuel when parked up.
> 
> Just a thought if you're diesel and intending to park up for a while.
> 
> Dougie.


I think Diesel is lighter than water.


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## AberdeenAngus (Jul 26, 2008)

Drag increases with the square of speed, and therefore the power needed to push an object through air increases with the cube of the velocity. If a car cruising on the highway at 80 km/h requires 30 kilowatts to overcome air drag, that same car will require 240 kilowatts at a speed of 160 km/h. 

Thus, a vehicle needs 8 times the engine power to reach twice the speed. In principle, this means that fuel consumption will increase fourfold (not eightfold, because the faster vehicle exerts the power only over half the time).

Of course these ratios are only true if there are no other losses e.g. the engine is perfectly efficient. But in general, as we all know, the faster you go the more fuel you burn.

I got a great demo of this the other week. With nothing in the way I was able to zip along a particular stretch of road at 60 (honestly officer). Coming back the same way, got stuck behind a JCB at 30.
MPG gauge was reading a smidge over 20mpg at 60 but 30mpg at 30.

At £1.20 a gallon, I know which I prefer.

So, I say, slow it down and let the cars past regularly (unless its Clarkson of course).


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## aultymer (Jun 20, 2006)

> asprn wrote: ‹ Select › ‹ Expand ›
> 
> I learnt something from a guy who was on the next pitch to us at La Manga last March (2007). He said that he arrived with an empty tank, but filled it to the absolute brim at the service station just along the road before pitching up for several months over winter. It apparently stopped water forming on top of the fuel when parked up.
> 
> ...





> I think Diesel is lighter than water.


The man from La Manga is right!!

Diesel is lighter than water - the reason for filling tanks up when the van is unused is to reduce the cold surface area of the inside of the fuel tank. This in turn reduces the amount of condensation forming in the tank.

It is well known in the boating industry where boats are left for long periods through the winter.


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## Freddiebooks (Aug 9, 2008)

Dear Aultymer and Teemyob.

Just to let you know that i wasn't suggesting that i waste clean water. I only mean't i bin the grey stuff asap and generally fill up when i get to a site. Or at home before i set off if roughing it , if thats possible in some of these motorhomes that we all have taken such a shine too. 

Dear Gerhard and Everyone else,

That is the most interesting and fascinating answer. Thank you for taking the time to do so. And the same goes to you all who have set my fears to rest.

Thanks Again

Freddiebooks


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## Freddiebooks (Aug 9, 2008)

And special thanks to you too Aberdeen Angus,

I also drive and Astra van 1.7 dti 53reg. On the best run ever i got 600miles on 46 Litres of diesel. When i have the roof rack attached, i can lose 50 miles or more to the tank. 

Freddiebooks


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