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You cannot go by the odometer or the speedometer - you need to calibrate them by driving a known distance; using the mile markers or map software or GPS. The longer the drive the better. If the tires are larger than the previous ones and/or the speedometer/odometer is overestimating your speed then is likely under estimating the distance - and both of these will combine to lower the calculated MPG when you pump the tank full each time you fill up.
The onboard factory MPG meters almost always overestimate the MPG, but with larger tires, then it might not be off by as much?
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