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I changed tire size on my suburban and fuel economy was way off. I couldn't believe the difference till I realized my speedometer was off and thus my odometer was also off.
If you put larger height (outside diameter) tires on it will take less revolutions to go the same distance. Now your speedometer reads slow. Your odometer now reads less miles traveled per tank. Looks like your gas mileage went down drastically because of the amount of gas used per miles traveled, when you actually travelled more miles than the odometer reads and the effect was much less than you calculated. Clear as mud? When my speedo says 70 mph I'm actually traveling closer to 78 per my gps.
You can have a speedometer shop re-calibrate your speedometer after a tire size change if you want accurate economy readings.
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