Only ever fill your car with, say, £10-£15 at a time, but do it a couple of times a week
(Depending how practical this is for you)

You should save a good few quid over time from the weight difference compared with a full tank.
(MrOli is ugly, but in the morning you will be sober, Mon 10 Jan 2011, 11:42, 6 replies, latest was 14 years ago)

Time
But how much is your time worth? If you spend a few extra minutes a week standing in a queue to pay for your petrol, have you really gained? My father in law will drive miles to get 1p per litre off his petrol, but I reckon he doesn't save anything as he's driving so much further to get it - might has well go to his local garage and just use less petrol.
(Smale is stuffed, Mon 10 Jan 2011, 11:49, Reply)

This is true, it's got to be worth it
Reckon half a tank of fuel extra is probably the equivalent weight of an extra person in the car. I'd do some calcs, but I've no idea where to start...
(MrOli is ugly, but in the morning you will be sober, Mon 10 Jan 2011, 12:26, Reply)

Christ,
how big is your fuel tank? My Mk1 Laguna held just over 14.5 gallons, which works out about 105 pounds* or about the weight of a young teenager.
Even with a 2 litre engine there was a noticeable difference in acceleration. I always ran mine about a quarter tank unless I was going on a drive in the country.

Good ways to save fuel is to make sure your tyre pressure is correct and you're not carrying a load of stuff in the boot that you don't need to.

And it's bollocks that having the air-con on is more economic than opening the windows.

* I didn't realise that petrol is less than 3/4 the density of water.
( sandettie light vessel automatic New Twitter - @bollocksreally, Mon 10 Jan 2011, 13:03, Reply)

Ah, didn't realise it was quite so light
Tis only a fiesta, but yeah, massive difference in acceleration between full and empty tank.

The bodies in the boot are so decomposed now, they weigh almost nothing.
(MrOli is ugly, but in the morning you will be sober, Mon 10 Jan 2011, 13:10, Reply)

And it's bollocks that having the air-con on is more economic than opening the windows
That's definitely true, if the air-con draws energy from the engine. You just have to accelerate more to power the air-con, so it uses fractionally more fuel, which negates the drag caused by the open windows.
(IHateSprouts is baking a huge pie shaped like an enormous pie, Tue 11 Jan 2011, 20:45, Reply)

At low speeds, yes
But above about 60mph, the drag caused on the engine by the AC pump is pretty negligible compared to the drag caused by having a window open.
Not to mention it's hardly pleasant driving down the motorway with a window down.
Also many air con units need to be run periodically, to circulate the refrigerant and lubricate the seal on the pump. If it is allowed to dry out through lack of use, you will often find that when you do want it, you can't tell the difference in cooling because it's unable to pressurize.
Most car owners manuals (ime) have a small note telling you to run the AC periodically during winter, for just this reason.
(Hairy Bob, Fri 14 Jan 2011, 10:45, Reply)