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I put on larger Goodyear eagle HP 235-55-16 vs the stock eagle GTII 225-60-16. My highway MPG has dropped approx. 4-5. The tires are running 37psi and the alignment is correct. Anyone else have this issue?

W.Cup
 

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wider tire = wider contact patch = more friction = lower gas milage...

But look on the bright side, you get better traction....
 

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You've decreased your overall diameter by 1/2 inch, (26.1" vs. 26.6") so you're speedometer/odometer is now incorrect by a certain percentage. There are sites that will calculate the amount of error. And by decreasing the overall diameter (and circumference) your tire now has to travel more revolutions to go the same distance. It’s just like changing gears to a higher number. It’s not really that much, but combined with the odometer error it can be really misleading.

The increased "rolling resistance" shouldn't hurt your gas mileage that much (especially with 37 lbs).

So you're probably still right where you were, or maybe 0.1 or 1 mpg less (IMO). :thumbsup:
 

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To expand on what Don said....your previous tires turned 758.19 times per mile. Your new tires turn 772.72 times per mile. This equates to 290.6 revoulutions more in 20 miles, or just about 1/3 of a mile less in 20 miles.

Your speedometer is reading 1.7% too fast.

When your speedo says 60, you are actually going 59.
 

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I switched from 215.65.15's to 255.50.17 and my mpg went way down. I used to get over 300 miles before the red "check gauges light came on" So, are you saying since I have an Inject Tech tuned pcm, I can get my new tire size reburned into the program and this problem will be resolved?
 

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No, it has nothing to do with telling the computer your tire size. The stock program has the car running fairly rich. They can lean it out some and get you some better mileage. It is a give and take, not a correction.

John
 
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