Tank Half Full

Idiophreak

Hi All,

I've been wondering whether only ever having your tank half full, whilst getting you better economy, actually saves you money?

Wonder if anyone's maths is good enough to work this out...

Thinking about it, I "fill up" (halfway) one week, when petrol's 1.19/l - now, two weeks later, when I fill up, the cost is 1.29/l - say 8% more than if I'd put that half tank in two weeks ago with the first one...

Now, I'm not sure that adding, say, 25l of fuel to the weight of your car's going to make 8% difference - so will it actually still be cheaper to fill up all the way and take advantage of the lower fuel cost?

Any thoughts?

AdrianHi

I think it extremely difficult to work out what it costs you to carry approx. an extra 25kgs / 25L of petrol around, which is average say 12.5L because your using it.
It will be different for different engines, cars, driving routes and styles.

On the basis diesel has going up 1p per litre every 4 days since the 11th of June (I've been tracking it in a spreadsheet and using petrolprices.com prices) and I go 14 days on a whole tank I'm thinking doing the complete opposite and filling up when you are down to half a tank is going to be money saving.

mustrum_ridcully

Some say that the increased fuel evaporation you get with a half full tank cancels out the fuel economy advantages you gain by carrying around less weight.

nomoneytoday

If you give someone a lift, or carry some extra luggage in the boot it will cancel it out anyway

I personally stick in £20 - when its gone its gone and in goes another £20

Pew_Pew_Pew_Lasers!

mustrum_ridcully wrote: »

Some say that the increased fuel evaporation you get with a half full tank cancels out the fuel economy advantages you gain by carrying around less weight.

Fuel evaporates according to exposed surface area, not volume. Unless you have a very oddly shaped tank.

cyclonebri1

You lot must all be out and out optimists, in my view that tank would be half empty




On a serious note would the rate of evaporation be dependant on the surface area at the level the tank was at or the diameter of the vent pipe, which is common at all levels??

vikingaero

I would forget all this.

If you're happy to potter locally with half a tank then fine. If you need a full tank for a long range trip then fine. If you're shopping at a supermarket you may as well fill up because the next time you fill up the price will have risen by a few pence.

Hintza

The cheapest garage around here is 12 miles away, I fill up when I pass. Driving to the filling station with the added hassle and time involved means it's a fill to the brim every time.

I'm just waiting for the day it costs £100 to fill up then I'll cry.....:mad:

BillScarab

Hintza wrote: »

The cheapest garage around here is 12 miles away, I fill up when I pass. Driving to the filling station with the added hassle and time involved means it's a fill to the brim every time.

I'm just waiting for the day it costs £100 to fill up then I'll cry.....:mad:

Shouldn't be long now...

In fact thinking about it a few years ago I ran an old XJ6 and that had a 20 gallon tank. So that would be cost £100 to fill up now if I still had it.

davetrousers

AdrianHi wrote: »

25kgs / 25L of petrol

Petrol does not have the same density as water. A rough (and easy) approximation is that Petrol is 3/4 (three quarters) the weight of water.

So 25 litres of water would weigh 25 Kg but 25 litres of petrol would weigh 18.75 kg.

So if you have a 50 litre fuel tank, then when full the petrol will weigh 37.5 kg. In a car that weighs 1,500 kg, it's not massively significant.

cyclonebri1

On downside of keeping a full tank is that if it ever gets nicked you've got a lot further to go to pick it up, happened to a mate of mine who had a 300 mile round trip to retrieve his car. It had been abandoned out of fuel before he even new it was gone.

Always keep mine as low as I practically can even if it means a few more top ups. Paid dividends once when I inadvertently left the car unlocked and some lowlife decided to syphon the tank, no chance, he was caught in the act hopefully suffering fume ingestion:mad: :T