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Design engineers consider cab height, cab length, box length, fender well openings and many other appearance details for proper proportions. Tire size is another proportional feature that plays into how we perceive the appearance of a pickup truck. Most agree the factory tire size of 245/75R16 has always appeared to be too small. Nearly all agree - except that is, for GM. While some owners feel anything less than 35's are too small, the 265/75R16 has always been a popular tire size for a heavy-duty 3/4-ton pickup when considering both function and appearance. However, all of GM's 3/4-ton pickups are factory equipped with smallish 245/75R16 tires, and GM has gone so far as to deny drivetrain warranty coverage if your truck is equipped with anything other than 245's. That's just plain dumb.We can speculate that GM insists on only one tire size because it reduces the number of variables when troubleshooting a drivetrain problem, drivetrain programming depends on a correct input from the vehicle speed sensor for certified emissions control and drivetrain management, a shorter tire improves roll/cornering stability, and a standard tire size maintains designed engine torque and braking efficiency.
Those are good reasons, but people want what they want, and most want bigger tires. Designed power, emissions and braking are all important, and together would make a solid argument for GM's decision to insist that 2500HD owners only use 245's. However, that argument evaporates when you consider that GM's 3500 SRW (Single Rear Wheel one-ton) is factory equipped with LT265/75R16 tires. Except for modified rear spring packs to increase GVW, the 3500 SRW and the 2500HD are otherwise identical. In addition, GM's C4500/5500 medium-duty trucks equipped with the Duramax 6600 and Allison 1000 can be dealer programmed for different tire sizes and even differential gear changes. Why not the 2500HD? Hey, even the new for 2006 mid-size H3 Hummer gets 33's... The above photos show the appearance difference between the original 245's (upper photo) and 265's (lower photo), installed on the same truck. The 2001 GMC 2500HD shown here was purchased new (along with the Firestone 245's). At 46,000 miles, the time came for a replacement set, which coincided with a Michelin tire promotion at Costco.
What occurred during my visit to Costco shocked me, and resulted in this editorial. Costco apologetically refused to install a set of Michelin LTX 265/75R16 tires (or any tires of this size) on my 2500HD for two reasons. 1- GM can deny drivetrain warranty coverage if any size other than 245/75R16 tires are on the truck, and 2- Costco has been forced to replace a couple of Allison transmissions because the truck owners argued that the non 245/75R16 tires purchased and installed at Costco resulted in a transmission failure. This idiotic argument gained credibility only because of GM's position on tire size and drivetrain warranty. Claiming that 265's can damage an Allison is beyond ludicrous. Because of a couple of slimy truck owners, Costco was forced to create an absurd tire size policy for the GM 2500HD trucks. Tire specifications for the 245/75R16 indicate a 30.47" diameter and 95.72" circumference. Specs for the 265/75R16 are 31.65" in diameter and 99.43" in circumference.
This 3.7% difference in size results in a similar speedometer/odometer error. A 3-4% size difference doesn't sound like a lot, but there is a noticeable improvement in appearance. Truck owners want what they want, and 2500HD owners will likely continue to upsize their 2500HD tires, as I did. GM is apparently honoring the drivetrain warranty "if" the owner installs a set of OEM size tires before the service visit, and some less anal dealers agree with their customers regarding tire size, and honor the drivetrain warranty regardless of GM's official policy. A frank discussion with your dealer service manager before buying new tires could help to avoid a warranty denial problem on down the road. For those who choose to upsize their tires, there are a couple of products available that can be used to recalibrate your vehicle's speedometer/odometer. This is important to help maintain ABS (Anti-lock Braking System) calibration, to help with fuel economy calculations, and to help you drive at legal speeds.
For about $400, most of the available Programmers/Tuners available through the aftermarket offer the ability to recalibrate the vehicle speed sensor, and produce a more accurate speedometer/odometer. For about $275, Superlift offers their TruSpeed vehicle speed sensor calibrator that is spliced in-line with the vehicle speed sensor circuit located on the transmission tailshaft or transfer case. Superlift says: "The TruSpeed alters the speed signal at its source, so speedometer reading, ABS function, and transmission shift points are all corrected to work with any non-stock tire diameter. A simple four-wire hookup is all it takes to install the TruSpeed, and comprehensive installation instructions make calibration an easy one-man operation. It not only corrects the speedometer reading to within 1 percent, but it can be re-calibrated an infinite number of times if future gear ratio and/or tire changes are made to the vehicle." In a recent "GMC The Magazine", a periodical sent to all new truck buyers, GM said this in its "Tech Tutor" segment: "GM engineers work with the major tire manufacturers to develop a set of specifications that reflect customer wants and needs."