Car computer gas mileage

Mike, your example does not take into account engine loading which varies constantly in the real world. As one example, 2k rpms up a steep hill pulling a trailer is not the same as 2k rpms on flat ground. The throttle position alone tells you one scenario is consuming way more fuel for the same engine rpm compared to the other one.

In order to hit 1% as a SYSTEM, the sub-component tolerances and variations stack up must not total more than that 1%. I don’t believe the odometer alone is even better than 1% and that leaves a boat-load of other factors un-accounted for in the error budget as I mentioned previously.

@TwinTurbo: Yeah, I agree with you that R.C. overstates his case. In all fairness, I highly doubt he had any firsthand knowledge; I figure he’s parroting what an engineer told him. (We could have gotten a far more informative answer if the engineer answered directly, but I understand that’s not how the real world works.)

@MikeInNH: 'm still perplexed at how I’m “wrong” about on-board MPG being derived from injector duty cycles. I was told I was, in no uncertain terms.

I have had several Chrysler Corp vehicles with this feature. The first several tanks I did the math. It was always within .1 or .2 MPG of the computer’s calculation so I stopped bothering to do the math. This was the case regardless of terrain, season, etc.

There are seemingly endless blogs comparing computer MPG to “real” MPG. The consensus seems to be that the computers are about 5% optimistic, which also happens to be my experience.

I don’t see how the energy content of the gas matters. Both computers and gas dispensing pumps measure gallons.

I think that’s a point many people miss- the car is actually measuring very little. Gallons used is a calculation based on assumptions. Most systems only know the commanded pulse width of the injector. Everything is assumed to be in working order and follow some statistical average to derive an estimate of gallons consumed.

The ONLY problem that would be a big issue is…
Is the guage accurately reading the EMPTY mark ?
Is the computer’s ‘‘miles to empty’’ accurate or has it ever left you stranded ?

never rely on its estimate…estimate of mpg.

U should never run elec fuel pump dry. Driving with <1/4 tank is dumb. Why do people continue to drive with less? U see gas station, u stop. My point of filling tank to max, drive X mils, refill. Compare gas used on trip computer to actual gas added. Simple.