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I have a 99 quad cab 5.2L that is sporting 255/75 R16 instead of the stock 225/75 R16. The previous owner who put on these larger tires did nothing about reconfiguring the onboard computer, thus my fuel economy readout is inaccurate and I don't know how to refigure it without popping forty bucks at the dealer for them to reconfigure the computer. Also, is my speedometer still accurate? What about my odometer? What else could this affect?
 

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Well all the things you mentioned are out of whak with the different size tires. You can go the route of the dealer or go buy a tuner and adjust the tire perameters and then fix other performance items.
 

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As far as I know there arent any options to just change the calibration other then go to the dealer or find a tuner with this option.
A tool I like to use to find tire size difference is this:http://www.1010tires.com/tiresizecalculator.asp
It gives you the percentage of what your tire size difference is as a percentage I.E. 10% means when your speedo says 50 your really at 55
 
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Those are a couple of good suggestions!

The speedo and odo are certainly affected. Not sure that much else will be.
 

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I've got a '97 with a 5.2L. I get @ 12MPG no matter how I drive it (highway, city, different speeds). You can measure distances between filling up w/ a portable GPS unit & keeping a log each time you drive of how many miles. divide total miles by how many gallons you put in. Then compare to readout to see how off it is. There's a throttle body mod on this forum to get 2 more MPG, which I haven't tried yet. I'm pretty sure any air intake/exhaust mods would help also. This probably doesn't answer your question. Just my 2 cents.
 
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