Aprrox. how much fuel savings do you get if you only fill your tank half full?

Probably nothing when you offset it against the extra trip you made to go get fuel.

An extra 100 yards?

And as fuel rarely gets cheaper, it'll cost you more to put two half tanks in in two visits than to fill it right up in one :O)

On a practical level, the fuel in a, say, a 40 litre tank, is approximately 40kg (OK I know petrol is less dense than water, but its close enough and errs on the side of caution). So the half tank is only 20kg, or less than 2% of the weight of the car.

Assuming you run from half tank to nearly empty, or from full tank to nearly empty, that would mean that on average you would only be carrying 10kg more fuel, so we're down to 1% of the cars weight on average.

Again on crude approximation =A31 per litre and 1% extra weight =3D 1% less efficient (I know, but I'm not going anywhere the physics with the back of the imaginary envelope I'm using), then I think were somewhere in the region of 40p on the tankfull - best case.

Half a tankful.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Nick Dobb" saying something like:

Oh, about an extra 300mpg.

Well, that's if you're not efficient enough to stop on the way to a destination to get fuel on the same road, if available. But yes, a slight saving will be noticed on filling the fank just half-full, but not exactly a huge amount, as petrol is very much lighter than water thankfully.

how much lighter do you think it is?

cheers, clive

About three-quarters as heavy?

Chris

About 262kg/cu.m lighter at 20oC, per 1000.

Somewhere in the region of f*ck all. Carry a few bags of shopping home and you've negated any miniscule saving you might have made by carrying less weight.

Si

So if I put no petrol at all in my car the savings would be 'normous?

Si

I find that if I fill right up, I get roughly twice the miles per tank. Plus I only have to stop to fill up half as often.

Bear in mind too, that if you have to go into a filling station twice as often, you then have a higher chance of _having_ to pay more for your petrol (assuming you don't always go to the same station). Also, you may buy twice as many snacks, and then your weight gain will offset any petrol weight savings. ;-)

Z

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Mungo \"Two Sheds\" Toadfoot" saying something like:

Exactly! A car that sits in the drive can provide amazing economy.

Does that really count as "very much lighter"?

cheers, clive

I'll give it a try. If I get taxis to and from work instead of using the car I reckon I should save about £10-£15 a week.

Si

I was doing the same maths for a Ford Focus...

Weight about 1700KG Half tank about 20KG Fuel consumption around 40mpg

Ignore drag and assume all the fuel is burnt moving the weight...

20 * 100/1700 = 1.2%

1.2 x 40/100 = 0.48 mpg

but drag is probably significant making the saving worse.

In fact the mass is mostly significant during acceleration, so the saving will be very small indeed.

A Ford Focus, depending on model, weighs about 1270kg not 1700kg

You'd only save a quarter of a tank on average.

The equations for working out bhp requirement for a given weight, rolling resistance and drag are on my website. I have a further spreadsheet which translates that into mpg using average values for brake specific fuel consumption.

On a 55 litre Ford Focus tank the average weight saving by only half filling the tank would be about 9kg. Running the calculations shows the mpg saving at average steady cruising speeds between 50 and 70 mph would be about 0.1 of a mpg. At low speed a little more - about 0.2 mpg. At high speeds (>80 mph) it would be negligible because aero drag becomes dominant. During acceleration it would be proportional to weight saved and therefore around

0.2 mpg.

Total weighted average, depending on whether you do mainly long or short journeys, maybe between 0.1 and 0.15 mpg and saving about 60p every 1000 miles. However, every time you stop for petrol you waste a bit of fuel stopping and starting again, the engine cooling down a bit and driving in and out of the garage. With 7 or 8 fill ups per 1000 miles instead of 3 or 4 I guess you'd do little more than break even. If you had to go even 1 mile out of your way to fill up each time you'd be on the losing side.

I have another comparison. My own Ford Focus has averaged 30.1 mpg over the

10k miles I've had it. My old XR2i with about the same power but 300 kg lighter averaged 34.2 mpg over the same 10k miles and similar journeys. Ignoring engine size and other factors that comes to about 0.12 mpg per 9kg so my calcs above are maybe not far out.

In other words don't bother.

If i was biased, a third of a saving. As i'm not, only a quarter saving ;-)

I'm sure over time if you done many miles it would add up, but of course, why not just fill it full and be normal.