Fuel efficiency and Older cars : getting tank to empty before filling or not.

stuhse

On the days you are getting better MPG its because the car engine isnt having to work has hard for whatever reason (many have been given by previous posters).  Could be one journey was constantly into a head wind, next journey was with a constant tail wind.  If you've ever ridden a  bicycle in these circumstances you will appreciate the difference in effort required is huge.

born_again

s71hj said:

Whether the gauge looks appx half full, near empty etc plays no part in the calculations. The number of litres to get it back to full and the number of miles driven since it was previously full are the elements that are used for mpg calculation.

Clearly not getting what I'm saying.

So how do you calculate that you get better MPG in the bottom half of the tank?

Hint. You can't fill it to the top (which you keep saying you do)  As that only gives you the MPG for the whole tank.

paul_c123

prowla said:

I can drive my car for miles along the motorway and it shows more range than when I started.

Entirely logical and normal. The range is calculated by using the remaining fuel quantity measurement, multiplied by a weighted average fuel economy (more recent data is given more weight). So, if you're in a situation where the fuel economy used in a calculation is rising at a higher rate than the fuel quantity is dropping, naturally the range will go up not down.

After all, its simply a projection of a future scenario. The trip computer can't know the future so its an (educated) guess.

s71hj

born_again said:

s71hj said:

Whether the gauge looks appx half full, near empty etc plays no part in the calculations. The number of litres to get it back to full and the number of miles driven since it was previously full are the elements that are used for mpg calculation.

Clearly not getting what I'm saying.

So how do you calculate that you get better MPG in the bottom half of the tank?

Hint. You can't fill it to the top (which you keep saying you do)  As that only gives you the MPG for the whole tank.

I can comfortably do 400 ish miles before the flashing fuel light goes off therefore. I know I'm much nearer half if I've done 200 ish. I fill up until the garage petrol pump kicks me out a few times, getting it back to where it was when I last filled up and set the trip mileage to zero. What I was saying is I get a better mpg only using the "top half (ish) " of the tank if we're purring it that way.

paul_c123

So you're saying, you can do more miles if you use 2/3 of the tank, than you do if you use 1/3 of the tank?

s71hj

paul_c123 said:

So you're saying, you can do more miles if you use 2/3 of the tank, than you do if you use 1/3 of the tank?

Not more miles, more miles per gallon if i use half a tank or less from full compared with if I use pretty much a whole tank from full.

born_again

s71hj said:

born_again said:

s71hj said:

Whether the gauge looks appx half full, near empty etc plays no part in the calculations. The number of litres to get it back to full and the number of miles driven since it was previously full are the elements that are used for mpg calculation.

Clearly not getting what I'm saying.

So how do you calculate that you get better MPG in the bottom half of the tank?

Hint. You can't fill it to the top (which you keep saying you do)  As that only gives you the MPG for the whole tank.

I can comfortably do 400 ish miles before the flashing fuel light goes off therefore. I know I'm much nearer half if I've done 200 ish. I fill up until the garage petrol pump kicks me out a few times, getting it back to where it was when I last filled up and set the trip mileage to zero. What I was saying is I get a better mpg only using the "top half (ish) " of the tank if we're purring it that way.

So you actually do not know what the MPG is, you are just guessing.🤦‍♀️

MeteredOut

born_again said:

s71hj said:

born_again said:

s71hj said:

Whether the gauge looks appx half full, near empty etc plays no part in the calculations. The number of litres to get it back to full and the number of miles driven since it was previously full are the elements that are used for mpg calculation.

Clearly not getting what I'm saying.

So how do you calculate that you get better MPG in the bottom half of the tank?

Hint. You can't fill it to the top (which you keep saying you do)  As that only gives you the MPG for the whole tank.

I can comfortably do 400 ish miles before the flashing fuel light goes off therefore. I know I'm much nearer half if I've done 200 ish. I fill up until the garage petrol pump kicks me out a few times, getting it back to where it was when I last filled up and set the trip mileage to zero. What I was saying is I get a better mpg only using the "top half (ish) " of the tank if we're purring it that way.

So you actually do not know what the MPG is, you are just guessing.🤦‍♀️

I think that was obvious from post 1 when it was stated the MPG increased from 33mpg to 40mpg when the tank went from nearly empty to 2/3 full.

Grey_Critic

20 - 30 - 40 - 50 mpg It all depends on how and where you drive taking into account head/tail/cross winds and force of such. Traffic conditions plus time of day or night - Tyre pressures - besides vehicle weight during that time and of course speed. All play a part.

If you make the same journey each day at the same times you will probably be able to get a reasonable indication of MPG but without fitting an accurate meter to measure it then that is all. Weight DOES plays a part in this. 

Years ago motoring magazines used to try to get an accurate measure and it was done over a measured circuit trying to match REAL motoring conditions.

paul_c123

s71hj said:

paul_c123 said:

So you're saying, you can do more miles if you use 2/3 of the tank, than you do if you use 1/3 of the tank?

Not more miles, more miles per gallon if i use half a tank or less from full compared with if I use pretty much a whole tank from full.

But you're using more than half the tank - that's the whole point. The mpg is roughly the same in both halves of the tank, but the gauge is displaying "half" when its actually a little less than half (or, your "empty" isn't really empty, so the 2nd half isn't quite as much as you think it is).

Put simply: your fuel gauge isn't being interpreted properly.

You still haven't posted your raw data which we can use to check your assumptions and calculations.