Quote:
Originally Posted by jkp1187
Subscribed -- my wife's '06 Pontiac G6 GT has a similar problem -- usually it's optimistic by +2 MPG. Sometimes +3. (One time it was pessimistic, but that's the outlyer.)
I'll probably just get another Scangauge for her car one of these days....
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It might be more accurate to simply calculate a correction factor for the onboard reading. The vehicle unit generally has access to target AFR and injector pulse width, which the ScanGauge generally does not.
When you 'calibrate' and OBD-II based MPG gauge, you are not really calibrating for variation in sensors so much as variation in the driver and common routes. The unit has to make some big guesses, always on actual AFR, and often (if no MAF sensor) on variable things like VE. So you end up with a fixed setting guess, and a correction 'fudge' factor that was empirically derived.
That value will be wrong if driving habits change, or you just have to drive over hillier or flatter terrain than usual. Since it is a guess anyway, then it might be better to apply it to the built in, since it (at least theoretically) has access to some better data.
If you are interested in really good precision as a measurement (vs. a rough measurement and a 'you' specific fudge factor). I think that MAF + a fast wideband (preferably one that can go to free air), or, depending on your injectors, measuring IPW are the best ways to go. On modern cars measuring IPW is tricky, not electrically, but because of all the games they play with cyls, etc. In that case, MAF + wideband is probably the way to go.
On an older car, IPW is a great way to measure. I'd still use a wideband and MAF initially, to profile injector flow under different conditions, but that could be derived empirically with good accuracy.
-jjf