Re: Increased fuel economy - Saab 9000 Bulletin Board - Saabnet.com
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Re: Increased fuel economy
Posted by Eric Law [Email] (more from Eric Law) on Thu, 21 Aug 2003 09:18:33
In Reply to: Increased fuel economy, Montana Tom
[Profile/Gallery]
, Wed, 20 Aug 2003 20:48:49
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Tom,
Saw your post about the tailwind - I'm guessing you'd need a 40+ MPH wind to achieve a true 40 MPG. Have you by any chance cross-checked by figuring your gas purchases vs. trip mileage?
More likely it's an electronic issue. The computer calculates MPG by dividing miles driven by fuel used (duh!). Fuel use data comes via pulses from the Trionic ECU (or LH on an older car). Mileage data comes via pulses from the speedometer.
If the pulses get interrupted, the computer just assumes the number in question is zero. So if mileage pulses are missing, gas mileage will read low, or zero if the pulses are interrupted completely. Likewise, if the pulses from the Trionic are missing, MPG will read high, all the way up to "99" (the highest it can read) if it gets no pulses at all.
My '86 had this problem intermittently (never found the cause). If I was watching at the time, I'd see the instantaneous MPG rise for no apparent reason, often to "99". Throw a few "99"'s into the average MPG, and you end up with an erroneous high number.
Eric Law
posted by 63.173.23...
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