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Originally Posted by Fx4wannabe01
I think it'll be minor...very minor....as little as 2%. So minor, that I think you're needle is off just a hair and you shouldn't worry about it.
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I don't know, Shane -- I'm running 31x10.50's on my truck since it was lifted in May, and my speedo/odo are off by just a shade under 10%.
I arrived at this figure by comparing my MPH and ODO with my GPS on open, flat roadway with a 7 satellite lock (and, thus, a 15-foot circle of accuracy [which is about the length of my truck]). If I set the cruise at 70 MPH per the speedometer, the GPS reads 77 MPH; if I set the cruise at 40 MPH, the GPS reads 44 MPH.
Mathematical calculations (using odometer/trip-meter readings vs. gallons to fill the tank) bear this out: against previously observed MPG calculations and current (post-lift and larger tire installation) MPG calculations, there's a difference of 9.88%. This can be explained by the odometer (and, thus, speedometer) reading being off by 9.5 to 10% (allowing for marginal variation in computed MPG due to varying environmental factors, gasoline quality, driving conditions, etc).