Half tanks are more economical IMHO

[obi_waynne]

I did an experiment over the last few months and have concluded that half tank fills mean the car will be more economical.

Dec (Full tanks) 2 fills = 35.6, & 34.9 mpg (£1.03 per litre!)
Jan (half tanks) 5 fills = 37.2, 37.2 37.3 37 & 36.9 mpg
Feb (full tanks) 2 fills = 35.9 & 36 mpg
Mar (half tanks 3 fills = 37.9, 37.2 & 37.3 mpg -Long motorway run for the first half tank (£1.10! per litre!)

Driving was very mixed over the months but I did seem to run a couple of MPG better. Any else tried this? In theory the less weight means you get better economy as you are lugging less weight around.

This is not scientific but I'd be interested to see your results.
I filled up from when the petrol light came on and filled with 23 litres of fuel each time for a half tank.

 

[sbtUK]

It all seems to make sense to me

Assuming fuel has the same density as water (I know it's actually a bit less, but I can't be bothered to look it up ), you're looking at about an extra 1kg of weight for every litre of fuel. So a full tank of fuel (45-50l) is the equivalent of having a small-ish passenger in your car the whole time.

It's a pretty logical (and correct) assumtion that the less your car weighs the more economical it will be. So the less fuel you put in your car, the better the fuel economy.

Obviously, you've got to find a happy medium. Putting 5l of fuel in your car at a time will provide close to the best economy possible using only this method, but you'll be back and forth to the petrol station all the time, and probably cancel out all your good intentions in the process

On a related note: I'm reminded of my ex-girlfriend's confusion at how much fuel she was using whenever I came to see her (She lived quite far away, so I went to see her for a week at a time every now and again). As it turned out: she usually drove her 900cc Fiat with no-one else in the car. Me, being 6'5 and about 17 stone, caused her to get about 50 miles less on a full tank of fuel.

So it can have quite a big difference, but the difference is a lot more noticeable in low powered, lightweight cars

Good thinking, batman!

 

[Massive]

i dont get this when ppl say that cos surely if you fill your tank you last longer before having to fill it, (im guessing half tank is supposedly better cos its lighter on the car) is this is so then when you tank depletes to half the capacity from full surely you get the ecnomical advantages of having a half full tank anyway, doesnt make sense!

 

[obi_waynne]

This was what I was setting out to prove or disprove. Yes you need to fill up more often and if this requires a detour to the petrol station it will not mean much of a saving. But if like me you pass a petrol station it is not a problem filling up more frequently.

The idea is that you run the car in the lower half of the tank.

As you pointed out massive, a full tank is only full for the start of the journey. :???:

You will save in real terms 1/4 of the weight of a full tank of fuel (because my maths tells me that the full tank empties therfore losing 50% of the 50% weight saving - I'm prepared to stand corrected on this one.) If you have a small fuel tank there probably isn't much in it, but if you have a 70/80 litre tank this becomes an interesting excercise. My tank is about 45 litres and I managed to save a couple of MPG!

It would be interesting to see what happens if we fill up the tank each time it gets half empty - pushing this theory the other way. The lesson I think is to let the fuel level run right down before we fill it up again!

 

[Massive]

that might give a stronger arguement by filling it up when hallf full, but i see now how even having more frequent vistis as you never have the wieght of your full tank in the first place,

i did a little to try and save money on petrol last week, my petrol lasts about a week on average, sometime i drive slow, sometimes a bit reckless with high revs, last week i drove my car quite hard and found that it lasted just as long as driving normally, but this can have a bad effect on the car long term so i try to stay clear of this,

but now ive devised a plan to slash my pertrol money in half! i do a fair bit of cycling so im gonna cycle to work every other day (i would do it everyday but its like 18 miles one way) and then i can buy those angel eyes i wanted, by putting away the 40 quid i would normally spend on petrol for that week, so hopefully a tank should last roughly 2 weeks im hoping!

 

[HDi fun]

Why are we all of a sudden so paranoid about driving cars hard and causing damage? The most important thing is to service them hard.

We'll all be changing into top gear at 28mph next.

 

[obi_waynne]

I thought everyone changed into top gear at 28mph!

I'm just talking about fuel economy - no one said anything about not driving them hard! Cars are made to be driven but for the daily commute lets go eco friendly - then we can enjoy ourselves at the weekend with a clear conscience!

 

[Massive]

im not paranoid, it just cant be good for the car really can it, just stating it didnt make much difference to my fuel comsumption when i was driving hard and well normal

 

[HDi fun]

I agree, mine's much the same whether you boot it or nurse it.

 

[obi_waynne]

I thought I would bump this thread as it was an interesting exercise and our newer members might find it a good read or have comments to add to this debate.

 

[Dr Acula]

Well I think it is a load of cobblers. You cant tell me exactly how much fuel is 'half a tank' as you could be out by a litre quite easily, so that in it's self can give more or less mileage depending on whether you over or under estimate what is half full.

 

[obi_waynne]

Well I think it is a load of cobblers. You cant tell me exactly how much fuel is 'half a tank' as you could be out by a litre quite easily, so that in it's self can give more or less mileage depending on whether you over or under estimate what is half full.

Fair enough, but let me reassure you that I filled up with exactly 23 litres of fuel each time and calculated the fuel to mileage manually AND by using the trip computer which was reset each time and measures the actual fuel usage.

I'm pretty pedandic and scientific in my approach, short of having a lab!

NB: the fuel light comes on at the same point and in each case I drove no more than 1 mile to the petrol station to fill up so I'm pretty happy with the overall accuracy of this experiment - especially as trip computer readings were also primarily used.

Also doing it over a period of time shows that the half tanks were always offering better mpg.

The only concession I will make is that it's hard to say how many MPG you actually save, this depends on so many factors and things like it being colder on the half tank fill periods can skew this result.

Petrol at 23 litres weighs 17.25 Kg
Petrol at 46 litres weights 34.5 Kg

I think that 17kg of extra weight is significant - take out your spare wheel and see what this does to fuel economy and handling.

 

[Fabia17]

I removed my spare wheel to try this thing out, so off I go, my trip computer set, I hit the motorway as part of the test, shortly I got a blow out and I have been sat in the sun running the engine with aircon on full trying to stay calm and cool until the AA man arrives to tow me out, I've used loads of fuel..... I give up........ Buahahahaaa... Nope just kidding.... Honest, but I'm definitely that guy, should I have tried.... Anyhoo, its pretty logical, and quite reasonable. Do I do it...? Nope I'm a full to empty guy almost all of the time..... But great reading.