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was wondering if my mpg readout would be affected by larger rims and tires. I used to get 18 with my 265/70/17, but now it is showing 15. Could the computer just be off due to the size difference? I plan on just filling up and calculating that way, just wanted to see if anyone else had noticed this or had a quick answer. thanks
 

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I'm not sure what the computer is basing the calculation on, but I'd guess the miles plays a role in the calculation, so if your tire size is changed then the pcm needs to be calibrated.
 

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On older vehicles, the calibration had to be reset
I won't swear to it, but i believe the computer tries to adjust itself on the newer vehicles

I do know that the computer is capable of learning a drivers style of driving
A friend of mine lets his teenage son drive his Ram, the computer is so confused between the 2 driving styles, nothing seems to work right
 

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U need to get it re-calibrated, it should affect it. The only way that it wouldn't need to is if the way it calculates miles automatically via a radar or something. But it doesn't to my knowledge. On my 05 I had to and on any of the programmers one of the main functions is recalibrating for tire size. Leads me to believe u need to, can't think of a reason you wouldn't need to. Cost me 35 bucks at my local stealership.
 

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Larger diameter tires will affect the mileage, the same way that it will affect the speedo calibration. Other things that will affect the reading are things like idle/warm up time, plus you're engine now needs to turn a larger rotating mass, so the mpg will drop, especially if you do a lot of non-highway driving.
 
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