My MPG has gone down??

Ultrasonic

bigjl wrote: »

...as he ran the tank low there is a possibility of dirt that has long since been at the bottom of the tank getting dragged into the filter.

As was pointed out to me recently, I'm not sure this makes sense since the fuel is taken from the bottom of the tank all of the time (whether it is near full or empty).

bigjl

I think of it like this, if there are say a thousand little bit of dirt in the tank and the tank is full then they make up a very small percentage of the fuel being pulled into the filter.

When the fuel is almost empty the same thousand little bits of dirt take up a much more significant percentage of the fuel being drawn into the filter.

Sort of like the bottom of a proper cup of coffee or the bottom of a teapot made properly with leaves.

Ultrasonic

bigjl wrote: »

I think of it like this, if there are say a thousand little bit of dirt in the tank and the tank is full then they make up a very small percentage of the fuel being pulled into the filter.

It all depends whether the little bits are evenly distributed, or tend to settle near the bottom (as in your mug of tea example). To some extent this will depend how much the fuel is mixed up by the motion of driving about. At a steady motorway cruise I'd expect all of the bits to be near the bottom where the fuel is extracted, but perhaps less so for city driving (as is the case for the OP). As I said, somebody else pointed this out to me recently, but I can't really fault their logic.

bigjl

The only fault with their theory is that the fuel pickup doesn't sit completely flush with the tank floor.

Been a while sincd i have looked in a fuel tank though.


And when a car moves about the fuel will get pretty well agitated that means any dirt will be pretty well spread around the fuel.

Think of my teapot analogy again

If yiu look into it after letting the leaves settles, then gently swirl it about for a second or so snd i think they will be quickly evenly matched.

You would need a transparent container with a drain at the bottom to actually get evidence of my theory. But it would only tske some acceleration or braking for the fuel to really slosh about and mix.

If i was an engineering student again i would try to do this to see who is correct.

Though there wouldn't have been much call for it on a Civil Engineering degree!

Only wish i had finished it, lol.

Richard53

Using the fuel computer on my car I see a small difference between summer and winter. This summer I have had about 44 mpg average, compared to about 41-42 in winter. Not a huge difference, but it is there. But the fuel computer is a pretty blunt instrument compared with a proper brim-to-brim calculation (preferably over several tankfuls).

One thing I have noticed is that it takes a while for the fuel figure to reach a 'normal' steady number after I have reset it. If I reset it while I am driving (say up a hill), it will start off very low (and vice versa if I reset while coasting downhill), and it can take up to 100 miles before I see a figure that stabilises around something I am used to. If I reset the fuel average when filling up, and then drove fairly briskly, I would expect a much lower fuel figure for a while, until it stabilised. Have you pressed the reset button recently? Also, have you refilled the tank from another source and measured again (to eliminate a possible bad batch of fuel)?

Ultrasonic

bigjl wrote: »

Think of my teapot analogy again

If yiu look into it after letting the leaves settles, then gently swirl it about for a second or so snd i think they will be quickly evenly matched.

Yes, that was exactly what I was getting at above by drawing a distinction between steady motorway driving and stop-start city driving.

The fuel pump must draw fuel from the bottom of the tank or you'd never get all of the fuel out. Or I guess it could be set some small distance from the bottom to prevent the very dregs being removed?