Aftermarket wheel computer/warranty and fuel question - Ford Powerstroke Diesel Forum
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Old 05-10-2010, 08:17 PM
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Aftermarket wheel computer/warranty and fuel question

Hey there. I recently upgraded from my factory 17" sport rims with 265/70/17 to raceline renegade 8's with nitto terra grapplers 285/75/17's. I have not done any mods to the computer after adding the new wheels/tires. Does the computer upgrade make that big a difference. How do I go about changing it? Also if I take my truck in for warranty work, something trans related with the aftermarket rims void the warranty? My fuel mileage is suffering big time. Im averaging 10.4 on highway compared to around 15 with the smaller rims and tires. As you can see im new here, is there any warranty friendly mods to improve mpg's, or atleast a mod that can be hidden, maybe just a tune? Thxs
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Old 05-11-2010, 01:43 PM
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Is that mileage drop hand calculated, or the dash read out? You should not have that bad of drop with those tires. I have the 285/65/18 Terra Grapplers with stock rims, and am getting 15mpg mixed driving. Your warranty SHOULD be fine, but the dealer can always try to be an a$$ about things. Tuners such as SCT, H&S, Spartan are supposed to be undetectable to the dealership. A tune combined with an DPF/DOC delete should get you in the 18 mpg range.
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Old 05-11-2010, 09:44 PM
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Thats with the dash read out. I am kind of new to all this computer controlled stuff. Everyone around me says updating to match the tires is a must. Does it really make that big a difference? What changes? Shift points? What else??? Thxs
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Old 05-11-2010, 09:47 PM
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you didnt upgrade enough tire size to make a difference in anything.
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Old 05-12-2010, 05:16 AM
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Im not sure wait your referring to with the "computer", but you are fine, no other mods are needed. The first thing I would do is hand calculate your mileage, and see where you stand.
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Old 05-12-2010, 05:33 AM
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Cool Tire Diameter

The taller tire will show that you've driven less miles than you've actually driven. That alone will make your hand or computer calculations wrong. The 285/75's are 33.9" and the 265/70's are 31.5 - that's 2.4" difference in diameter. Your stock tires yielded 640 revolutions per mile and the new ones yield 595 revolutions per mile - that's significant! I had Ford plug in a new number when I bought my 35's - there were three places where it had to be changed. One for the transmission computer, and two places in the main computer. One fixes the speedometer/odometer, one adjusts the braking force for the taller tires. They charged me 1/2 hour to do it.

Last edited by jimd1050; 05-12-2010 at 06:20 AM. Reason: More info
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