Fuel Consumption varies on full/empty tank?

[hutchingsp]

Got a new 987 and noticed that if I keep the fuel tank pretty much full (i.e. top it up when it's half-empty) I get very slightly better fuel consumption according to the computer (which does seem to be fairly accurate) than if I let the tank run low before topping up.

I wondered if anyone had any idea why this would be?

Not a showstopper, just something I've noticed.

[GreenV8S]

Perhaps it's something wierd like the fuel being hotter / less dense when the tank is empty and the fuel flow sensor isn't compensating? If you log how much fuel you're actually putting in and compare this to the mileage, you might be able to see how acurate the computer readout is.

[porsche4life]

thats weird, would have thought the other way because of the weight.

[hutchingsp]

porsche4life said:

thats weird, would have thought the other way because of the weight.

So would I. We're only talking a few tenths of a MPG but it seems fairly consistent.

[willibetz]

I suspect the range computer is non-linear in its estimations.

While fuel density does vary significantly with temperature, the variation is compensated for by the lambda feedback loop and any problems with this would lead to poor running and warning lights.

[Vesuvius 996]

My 996 fuel gauge drops at a consistent rate until the lat quarter tank, then it seems to drop like a stone, to the extent that the first day I had it I nearly ran out!

[GreenV8S]

willibetz said:

I suspect the range computer is non-linear in its estimations.

While fuel density does vary significantly with temperature, the variation is compensated for by the lambda feedback loop and any problems with this would lead to poor running and warning lights.

Yes the engine management system will compensate for fuel density variations, but the trip computer may not. I don't know how it measures fuel consumption, but it may be from a volume flow sensor, which would probably NOT allow for density changes.

[willibetz]

Thanks, I misunderstood.

Edited by willibetz on Saturday 25th November 17:33