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Discussion Starter · #1 ·
Made an important discovery today-
I checked my odometer with a GPS phone app and found it reads way low by 30%!!
The car odometer reads 10 miles and the GPS data shows 12.9 miles
Considering that, all my MPG totals showing say 10 MPG actually are 12.9 MPG
Much better than I thought
This is USA gallons and 91 fuel 86 Spirit Calif delivery car with a -093 Altitude WUR
Its still is running rich by the color of the exhaust pipes so a rebuilt WUR is coming.
KJet w/ LAMDA "should " be burning near stochiometric 14/1 with a light tan color.
Mine is jet black and thick.

So if you have low MPG you might just do a GPS phone app check
Hope this helps someone
 

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That doesn't sound right at all, 30% is a big difference. I've not done exactly the same test but I drove 165 miles as per my odometer recently, and checking the route on Google Maps it's almost exactly the same. We might have different driving styles but I'm averaging a little over 17 mpg (UK gallons) this year so far, which is apparently around 14 using US gallons. For any mainland Europeans that's a little over 16 l/100km.
 

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Discussion Starter · #3 ·
Not using Google maps distances!
Using speedometer and odometer distances that the GPS actually traveled without a destination.

Car speedometer was noticed to be reading low so a GPS speed was checked and verified the speedo was showing low. As the odometer and the speedo in the car are concomitant an odometer check was done verifying the hypothesis.

Both reading low by about 30%
So any fuel MPG computations would be incorrect to true distances traveled
By using the actual travel distance for the MPG computation (down to an EPE of less than 50 feet in most GPS engines) the accurate MPG were discovered.

Longer GPS trips scheduled next week to reverify

With this in mind- Being that on the transmission we have drive gears for the speedometer calculations I can reduce the number of teeth on the driven gear (easy change out) by 2 less teeth (1 tooth is about a 2-2,5 MPH correction factor) to account for the 4-5 MPH slow speed reading.
That will also make the odometer closer to accurate but neither will be as accurate as a good GPS hack.
 

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OK, but if my odometer was 30% out it really wouldn't match up so well with the distance I know I've driven.

I don't use GPS while I'm driving, so I don't know how accurate the speedo is, I've been assuming that it reads a bit higher than the speed I'm actually doing as that often seems to be the case.
 

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Discussion Starter · #6 ·
My 86 Spirit has a mechanical odometer built into the speedometer
If the gearing is off on the transmission output then they both show incorrect readings.
Th gearing only is correct for a certain size (circumference) tire.
If the tire is changed the reading will be wrong.
IIRC its based on 1,000 revolutions/mile of the tire.
 

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After '89 both odometers are electronically driven by signal sourced in ABS module.
However they work independently. Electronic trip odometer, built in DIP, is configurable (miles/km).
 

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The gears in these odometers are notorious for crumbling away. Just changed mine a few months ago. Is it possible a replacement gear may have been put in at some point that wasn’t quite the precise size, but close enough to function almost okay? I bought a couple when I did mine from different places just to be sure because they were so cheap and one was defo a different size from the one I installed. Just a thought.
 
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