How does the MPG guage work?

[camaroman]

Just out of curiousity, how does that MPG guage under the Tach work? Mine "stuck" above 40 MPG's briefly today, I got it "unstuck" again byt turning the ignition off and then turning back on.

I'm just curious how that little guage works; I've never understood how it operates.

[Jared at Pelican Parts]

Im not entirely sure, but I think it operates based on fuzzy logic, speedometer operation and the inductive pickup on the ignition loom.

I kinda doubt how accurate it really is anyway. The same with the fuel remaining and MPG functions on the computer.

[camaroman]

ahh. I don't think it's entirely accurate, either; but I know it helps me keep my foot light on the pedal a bit more, ha ha. This is the 1st car I've ever owned that I haven't been harsh on when it comes to accelerating, and that stupid little guage is the main reason why...gas isn't cheap anymore, ha ha...

[1100]

It uses speedometer and fuel injection info to calculate consumption I belive. Yo uwill find it pretty close to what the computer calculates instantaneously. Not that this means that either will be accurate!

[carolinus]

1100 is right....the fuel injecton computer knows how much fuel is going in on each injector pulse, so it knows how much is being squirted in. It then divides your MPH by that rate.

However, the fuel input is always off because of clogged injectors, and the speedo is usually wrong by 3 or more MPH on these cars.

[clevy70911T]

Funny I was just wondering that myself the other day. I noticed what appeared to be a higher fuel consumption while it was cold than when warm at say 5th gear and 2500 rpm. As it warmed up , the "mileage" increased as I would expect. I also figured it had alittle to do with manifold pressure. My OBC sometimes sticks on 6.3 mpg and other times works fine. I have been tempted to carry a fuel can and see how accurate the "miles remaining" figure is.

[rgarling]

the fuel input is always off because of clogged injectors, and the speedo is usually wrong by 3 or more MPH on these cars.

the clogged injectors would cause a problem if the flow is restricted; however, the car does know how far (and thus how fast) it is going. It is just that the speedo is intentionally brain damaged to read too high.

[bertible]

Speaking of Speedo accuracy. I just took the snowies off my car, and was comparing my 205/60-14 summer tires to the 195/65-14 winters. I thought they were the same outside diameter, but not even close! Obviously this must have a large effect on the speedo too. The smaller size 205/60's (especially with 75% of tread gone) will cause a much higher reading than the larger winters.



I was wondering why everyone was driving so much slower over the last few months. It turns out I was breezing by them because I was driving faster!

Does anyone know what effective outside diameter of tire the speedo would be correct? I realize that you can't simply measure a tire statically to determine the effective o.d. (since it compresses when weighted and running) but I figure there must be a nominal size that the speedo was designed for.

[bertible]

I should add that the gas mileage I've seen on my computer is definitely better than my calculation using the trip odometer reading. My computer says 9.8 liter per 100 km (about 23mpg US) but my own calculation shows about 10% less. This means that the speedo error isn't the only thing that causes the high mileage. Although some have attributed injector clogging to this error, wouldn't clogged injectors cause slower fuel flow?

[Jared at Pelican Parts]

I thik that ther is a pre-determined margin of error programmed into the computer to account for things like tire inflation, injectors, etc..