Saving fuel myth?

[pete75]

There are a lot of fuel saving ideas about at the moment. One frequently mentioned to fill the tank half full rather to the brim so you're moving less weight. Can't see that ir makes any difference. If your car needs 100 litres of fuel to travel 1000 miles then you've shifted 100 litres. This is true if you run half full to empty or full to empty.

[Jdsk]

Consumption goes up with mass. Proportional for acceleration and ascent. Some effect on rolling resistance. (Very little effect on aerodynamic resistance.)

Jonathan

[Paulatic]

The theory is you’re carrying less fuel/weight and therefore a lighter vehicle gets better mpg.
The reality is do you notice less mpg when carrying a passenger?
Have you really got nothing better to do than stopping more frequently at service stations?
You realise when prices rise that tank of fuel is better than money in the bank.

[rjb]

I've seen that mentioned on the MSE page https://www.moneysavingexpert.com/recla ... uel/#drive
If you have a large tank then there's probably some merit in it but as I have a small car with a 35 litre tank it's probably not going to make much difference.
As cyclists we are generally in a better position to judge how we drive as anticipation is one of the biggest fuel saving tricks around.

[CliveyT]

How much extra fuel do you lose driving into a petrol station to fill up? Maybe if your route is right past one and you can pull in without queuing .

In the same way about people who drive out of their way to go and get cheaper fuel- if it takes you 10 minutes longer to save £1 then that's less than the minimum wage (and uses more fuel)

[PedallingSquares]

I just bung £40 every time it needs it.My fuel warning comes on at 55-75miles left and I put some in then.Sometimes it lasts a week,sometimes a couple of days.£40 is just a figure I've used for years. Unfortunately these days it's less than half a tank
I only ever fill it up if we're going on a long journey like holidays etc.

[pete75]

The tank on my car holds 68 litres. I usually fill it up with 60 litres. That's enough for about 740 miles. I'm transporting a total of 60 litres of fuel to do those miles. My point is it should make no difference to fuel consumption if I fill up once with 60 litres or twice with 30 litres as I've transported the same weight of fuel. Money saving experts are claiming I'd use less fuel by filling up twice.

[Paulatic]

The tank on my car holds 68 litres. I usually fill it up with 60 litres. That's enough for about 740 miles. I'm transporting a total of 60 litres of fuel to do those miles. My point is it should make no difference to fuel consumption if I fill up once with 60 litres or twice with 30 litres as I've transported the same weight of fuel. Money saving experts are claiming I'd use less fuel by filling up twice.

Using your figures you’ve travelled 370 mls carrying more than 30 litres. That’s approx 25kg getting less all the time. Experts reckon you are now going to get more mpg in the next 370 mls because you’re carrying less than 30 litres or 25kg less weight.
Experts? Pha! An Ex is a has been and a spurt is a drip under pressure.

Mind you another expert says

The EPA says that for every 100 pounds taken out of the vehicle, the fuel economy is increased by 1-2 percent.

https://www.autoblog.com/2009/10/29/gre ... fficiency/

Don’t carry any fuel and you’ll get great mpg. I think that might be flawed

[Jdsk]

The tank on my car holds 68 litres. I usually fill it up with 60 litres. That's enough for about 740 miles. I'm transporting a total of 60 litres of fuel to do those miles. My point is it should make no difference to fuel consumption if I fill up once with 60 litres or twice with 30 litres as I've transported the same weight of fuel. Money saving experts are claiming I'd use less fuel by filling up twice.

And they're right, other things being equal. In one scenario the vehicle and its contents are less massive than in the other and the consumption will be lower.

The effect may not be very great.

Jonathan

[reohn2]

I fill up where it's cheapest,currently Sainsburys @ £1.51 per litre,if there's any difference in mpg when I've been forced to put half a tank or less at other more expensive stations I haven't noticed and I do keep a regular check.

I've also tried the super fuel sold by Shell and BP and found no difference in mpg or my vehicles running any better than on the cheap stuff.

EDITED to add the bold type

[axel_knutt]

The tank on my car holds 68 litres. I usually fill it up with 60 litres. That's enough for about 740 miles. I'm transporting a total of 60 litres of fuel to do those miles. My point is it should make no difference to fuel consumption if I fill up once with 60 litres or twice with 30 litres as I've transported the same weight of fuel. Money saving experts are claiming I'd use less fuel by filling up twice.

If you use 60 litres over 740 miles then the average tank contents over that distance is 38 litres. It you fill up twice then the tank contents averaged over the same distance is 23 litres.

[Jdsk]

The tank on my car holds 68 litres. I usually fill it up with 60 litres. That's enough for about 740 miles. I'm transporting a total of 60 litres of fuel to do those miles. My point is it should make no difference to fuel consumption if I fill up once with 60 litres or twice with 30 litres as I've transported the same weight of fuel. Money saving experts are claiming I'd use less fuel by filling up twice.

If you use 60 litres over 740 miles then the average tank contents over that distance is 38 litres. It you fill up twice then the tank contents averaged over the same distance is 23 litres.

And that's why the mass is different between the two scenarios.

Jonathan

[pete75]

The tank on my car holds 68 litres. I usually fill it up with 60 litres. That's enough for about 740 miles. I'm transporting a total of 60 litres of fuel to do those miles. My point is it should make no difference to fuel consumption if I fill up once with 60 litres or twice with 30 litres as I've transported the same weight of fuel. Money saving experts are claiming I'd use less fuel by filling up twice.

And they're right, other things being equal. In one scenario the vehicle and its contents are less massive than in the other and the consumption will be lower.

The effect may not be very great.

Jonathan

In both scenarios a total of 60 litres of fuel is being shifted ie the energy used will be 60 * whatever it uses to transport a litre of fuel. They don't take into account any consumption associated with filling up either. That may well increase the fuel usage from following their advice. The price of fuel may increase between fill ups too.

[Jdsk]

The tank on my car holds 68 litres. I usually fill it up with 60 litres. That's enough for about 740 miles. I'm transporting a total of 60 litres of fuel to do those miles. My point is it should make no difference to fuel consumption if I fill up once with 60 litres or twice with 30 litres as I've transported the same weight of fuel. Money saving experts are claiming I'd use less fuel by filling up twice.

And they're right, other things being equal. In one scenario the vehicle and its contents are less massive than in the other and the consumption will be lower.

The effect may not be very great.

In both scenarios a total of 60 litres of fuel is being shifted ie the energy used will be 60 * whatever it uses to transport a litre of fuel.

The "total" volume shifted doesn't determine the mass throughout the journey or the energy required to move it. axel_knutt has kindly added the numbers and a picture.

Jonathan

[thirdcrank]

Purely on the matter of physics, as has already been painstakingly explained, the more weight being shifted, the more fuel will be consumed. AIUI, this as part of the rationale for so many new vehicles being supplied without a spare wheel as that keeps the weight down for the official tests.

Beyond that, it seems obvious that the weight of fuel in a vehicle drops throughout a journey so if for ease of calculation you fill up at the start of a journey and run out at the end, then the average weight carried will be half a tankful. Using the same working out, doing the same working out, starting with a tank half full and refilling halfway would mean your average weight of the fuel load would be a quater of a full tank