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Old 09-20-2012, 07:10 PM
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Odometer reading off

So, I bought my truck used, with larger than factory tires on it, and now I'm confused.

I noticed when driving to Dallas in my (stock) BMW that the mileage was 248. When I drove the truck a couple weeks ago, the mileage was 221 (yes, same route... . This caused me to call into question my odometer reading, and also my fuel economy. If this is true, my odometer reading is low, and the fuel economy I'm showing is also low (at least using the in-dash computer, not by calculating by hand). The strange thing is that the speedometer is correct, at least according to driving next to my brother, and the LED speed things on the road.

Am I missing something here? Does anyone know how the mileage and speed are computed into the dash? I would have thought it would be axle ratio/tranny ratios, etc, computed from a preprogrammed tire size, which would be fine if the speedo was also off... Any thoughts?
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Old 09-21-2012, 09:25 PM
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bump. Any help?
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Old 09-22-2012, 09:00 AM
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It all works out to how many revolutions your tires make in a mile. The system just counts how many times the drive shaft (or wherever it's monitoring) turns, then calculates how far you've traveled. Taller tires will show less mileage than actual (you travel farther for each revolution), shorter tires will show more mileage than actual.

Dealers can reprogram the system to accommodate the different tire revs per mile, I'm sure other shops with programmers could too. Tire specs for each brand/size will list the revs per mile.
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Old 09-22-2012, 09:15 AM
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Quote:
Originally Posted by Hopeless Diamond View Post
It all works out to how many revolutions your tires make in a mile. The system just counts how many times the drive shaft (or wherever it's monitoring) turns, then calculates how far you've traveled. Taller tires will show less mileage than actual (you travel farther for each revolution), shorter tires will show more mileage than actual.

Dealers can reprogram the system to accommodate the different tire revs per mile, I'm sure other shops with programmers could too. Tire specs for each brand/size will list the revs per mile.
Right. But that doesn't explain why the speedometer is correct, but the odometer/fuel economy are not.
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Old 09-22-2012, 12:14 PM
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Good point. I would have thought they are tied together but being electronic, they don't have to be. I know my speedometer is off by a few miles an hour, slow (compared to a GPS). Have never verified actual odometer distance (yet), but I do get the sensation it's not real accurate and the mileage calcs aren't right. I just did a ~750 mile trip, first part it calced just over 21.5 MPG and I went~425 miles before I refilled. After refill it calced barely 20, but was almost empty after 330 miles. I didn't pay attention as much as I should have on the fill-up but I know it was way more than the 16-1/2 gallons that that would have calculated out to (I think I also figured out why that station was so much cheaper than others in town)
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