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Hey guys i remember something about this from a long time ago but im wondering if anyone has found a work around. I recently put 35" tires on my 2014 2500HD Cummins. When i went on my Bully dog tuner/moniter and adjusted the size to 35 inch i got a check engine light. the only way i could get it to go away was to stop, turn the truck off, and go into run mode to switch the size back to 34 and 3/4. any help would be appreciated.
 

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Don’t know what this bullydog thing is, but most 35” tires don’t measure 35” even, what tire brand and name are you running? Start at 33.5”, take the truck for a ride with GPS on hand, and check your speedo matches the gps, adjust the size up and down accordingly, don’t worry about the number youre putting in the calibration, what matters is your speedo actually matching your gps.
 

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Most manufacturers will publish the exact diameter of their tires in a given size
 

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Yes, but that's tire unmounted without vehicle weight, that's not the diameter built in the factory calibration.

For example, take a GY Wrangler HP tire in factory 275/60/20 size, Gy advertises the tire as 33", same for the SR-A's in the same size, and more likely like any 275/60/20 tire out there, though some manufacturers will advertise lil smaller or taller, don't know why they don't have the same standard across the board.

Factory calibration for a RAM 1500 crew cab for instance, that came with 275/60/20 is 32.21" in year 2014 with the 6 speed trans, which is closer to what that size measures mounted with weight. i'd have to look at newer trucks with the 8 speed trans, but i don't expect this number to be .xx different assuming the same 275/60/20 factory tires.

We used to go over the same thing in the Jeep forums, there is a few hand of manufacturers that will make their tires very close to true or true to their size, ex: Maxxis Creepy Crawlers, Interco Irok etc, but for the most part, most are smaller than advertised, then you have count for vehicle weight.

This is one of the best examples i've seeing out there to measure a tire and compare that tire to others,
https://wayalife.com/showthread.php/35075-The-Ultimate-TRUE-TIRE-SIZE-Database

I've been using this measurement method for years to calibrate my speedo when i change to bigger tires, i measure to the edge of the shoulder, and not at the top of the thread, what i get in size, i enter in the calibration, and my speedo matches my GPS everytime first time.

For instance, I'm running Falken AT3W in 315/70/17, 33.375" measured at the edge of the shoulder under my RAM 1500 crew cab weight, at 45PSI, Falken advertised the tire as 34.6", or 34.8" for the same tire in 35x12.5x17.

I've had KO2's in the same 315/70/17 size in my JKU, measured 33.5" at the shoulder, mounted, at 30 PSI, BFG advertises the tire at 34.4", same tire in 35x12.5x17 is advertised at 34.5"

The one in the left(shoe in pic), is the 35x12.50x17, the one in the right is the 315/70/17, they're both BFG KO2's, took this picture to show others the difference between both sizes

They have pretty much the same height and width unmounted, at least to the naked eye, the 315 is just a tad wider though

20160220_093928 by SilverFJ RSM, on Flickr

Height, the 35x12.5 is perhaps 1/8" taller
20160220_093914 by SilverFJ RSM, on Flickr

When i want a 35, i go with 315's, they're cheaper, and the height difference is not enough for me to pay more just t have the number 35 stamped in the sidewall :)

Hope it helps.
 

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Yeah that's what I am saying. Nitto lists the E range 275/60/20 Ridge Grappler as 33.23" diameter and BFG lists the 275/60/20 KO2 as 33.0"


They're both 275/60/20s but the actual size is slightly different. Part of that depends on the width of the rim its mounted on, which is one reason they use section widths and aspect ratios instead of outright sidewall heights and tire widths for measurements.


Manufacturers will also publish revs/mile.


GPS has a positional margin of error and accuracy will depend on the model
 

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if i'm understanding you correctly, you're saying to go with the manufacturer published diameter spec, and don't worry using a GPS to check ?

if youre trying to acurately correct for bigger tires and don't want/cant measure your new tires, you could use advertised diameter -1, and use that as the your starting point, recheck with GPS when possible and adjust accordingly.

Sure any GPS have a marging of error, but i'm sure that we should be able trust any "working" GPS device to accurately pin point our speed, especially when you think a LEO can and will use, speed gun to check you're calibrating your tires correctly at any point in time :)
 

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Yes that's what I'm saying. Speedometers from the factory with the OEM tire combination aren't spot on accurate, and as your tread wears the accuracy of the calibration will change.


I guess you could be OCD about it, but not something I'd spend my time on
 
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