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Discussion Starter #1
What happens to Mileage if I go from 245/75/16 to 265/75/16?

How to you adjust the speedo to compensate for the slightly higher gearing that would result?
 

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Mine dropped a little bit, by about 1mpg. Winter arrived shortly after I moved up to the 265s so my numbers may be a bit sckewed.

You can use a programmer (Hypertech, Predator, etc...) or EFI Live to reprogram to the new tire size.
 

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Calculated mileage would be lower....245/75 to 265/75 is about a 3.7% difference in diameter. Multiply your odometer mileage by 3.7 and/or your mpg figure by 3.7.
 

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mr_udy;1583266; said:
Calculated mileage would be lower....245/75 to 265/75 is about a 3.7% difference in diameter. Multiply your odometer mileage by 3.7 and/or your mpg figure by 3.7.
Correct! Larger tires = better MPG; its akin to lowering your rear diff ratio.
 

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Discussion Starter #5
Bigger tires

Bigger tires will make it higher geared so your engine rpm will be lower at the same speed if you have the Speedo reset. But it will take more Torque to turn them.
 

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The dealer can reprogram you to a 265 too.
 

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I wouldnt worry about mpg with a diesel its just them gas jobs u have to wacth out for i no on my 01 dodge i went from getting 15 16 mpg to getting 6 mpg when i put my 38 inch boggers on it
 

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When your speedometer reads 100, you'll be going 104 mph. Either way the trooper gives you a ticket.

Tell me when you "glance" at the speedo going 65 if you care or can tell your going 67.4.


It just does not make that much difference. :shrug
 

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Discussion Starter #9
No

But to calculate your mileage the Odometer needs to be right.
 

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I'm running a brand new set of stock 245's on my truck at the moment. I checked my speedometer with my GPS and it actually reads a couple mph fast.

I think going to a 265 in my particular case will make my speedo/odo more accurate. Has anyone else checked this?
 

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mine was more acurate with 265's but had dealer recal for 285's when i switched, a yes you will loose milage. 1mpg for 265 2-3for 285's, search for some milage threads, and you will find those are averages the question is if you care enough to drive around on pizza cutters???????????
 

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Discussion Starter #13
OK Thanks

I think I'll stay with 245s and get some Michelins. the ones I have should last another year, I hope.
 

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MLA: How'd the dealer correct for 285s? GM tool only goes up to 265s...
 

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mr_udy;1584505; said:
MLA: How'd the dealer correct for 285s? GM tool only goes up to 265s...
Just a guess here, but if putting 265's on with the stock calibration of 245 brings the speedo closer to accurate, then if GM recalibrated the computer for 265's perhaps the speedo would be close with 285's.:idea:

I have also heard that dealers can only calibrate up to 265's, but never persoanlly asked my dealer service dept about it.
 

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I have read on here a few times that putting 265's on doesnt require a recal. Even though i have the Predator, i left mine alone... I dont mind if it reads 1-3 mph slow... I would be upset if it was reading 1-3 mph FAST and i was putting "extra" miles on my odo!!! MPG doesnt change that much by a 2-3% correction factor anyway. does it really matter if you are getting 16 mpg or 16.4 mpg?!?!

IMO if you go to 265's, leave it alone, if you go to 285's, calibrate for 265's...
 

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When I went to 265's the speedo now reads correctly, before (245's) it was off by ~2.5 mph at 55 mph.

DEWFPO
 

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I'm actually running 285s with 265 cal...simply cause I couldnt recal for the 285s... How do you know how accurate you are? I know most cars are off a bit...motor trend or car and driver used to print out spedo error for cars they road tested...dont know if they still do. I'd probably agree, that I'm not far off at all...but would like to know for sure.
 

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mr_udy;1585906; said:
I'm actually running 285s with 265 cal...simply cause I couldnt recal for the 285s... How do you know how accurate you are? I know most cars are off a bit...motor trend or car and driver used to print out spedo error for cars they road tested...dont know if they still do. I'd probably agree, that I'm not far off at all...but would like to know for sure.
The easiest way is with a GPS. The redneck way is to set your cruise on 60 and use a stopwatch to time yourself between mile markers. At 60 mph, you are travelling one mile per minute, so if your stopwatch says otherwise, you know your speedometer is off. For example, if it takes 57 seconds to travel between mile posts, then your speedo is reading about 5% (3 mph at this speed) slower than your actual speed. Since mile markers are not necessarily dead nuts accurate, you'll want to do this over several miles and take an average.
 

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There's two places on our highways out here that have radar guns that tell you your speed if your traveling 6 mph or over (the curve speed limit) going into these tight mountain curves.

Also, when I lived in NJ, they had mile markers every 1/10th of a mile. It was an easy way to check the odometer. If the ododmeter was off, I assumed the speedometer was off. Not very accurate but a ballpark non the less.

DEWFPO
 
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