I am increasing the size of the tires on my 04 duramax from 245's to 285's. I was told that I might need to reprogram the tranny? Has anyone had to do this? I suppose it has to do with the rear end gears as well. Its a great transmission and I don't want to screw it up just to make my truck look and stand better. Is there a website that you can find out with the truck VIn# what gears you have in the rear end? Appreciate the help.

As far as I know the Duramax only comes with 3.73 gears. I see that you haven't filled out a signature yet so I don't know if you have any kind of programmer for your truck. If you don't, you'll want to get one that can correct your speedo for you such as a Predator or EFI Live. I think GM can do it for you, but maybe only for up to 265's I think. I have 285's on my truck and I don't have mine calibrated since my Bully Dog won't do it. I have a Predator that will, but it won't work since I use the Bully Dog.

The allys recalibrate every 40 starts or when you turn the key to start. I've never heard about needing to recalibrate the tranny for tire size, they shift according to throttle position and engine speed.

I would say that you don't need to do anything other than drive it for a while and it'll adjust itself.

That's the beutie of these trannys, they're constantly adjusting to the way you drive.

Is he worried about the speedo or just the tranny shifting right? I thought with the way he was talking about bigger tires and rear end gears, he was worried about the speedo maybe. Maybe not.

Moving to Wheels and Tires

Is he worried about the speedo or just the tranny shifting right? I thought with the way he was talking about bigger tires and rear end gears, he was worried about the speedo maybe. Maybe not.

Don't care about the speedo. But wasn't sure how much the tranny was linked to it. I just dont want to srew up the tranny. Its been great so far and I hate to do something that I will regret. Spoke to a couple of mechanics that said I might change the way it shifts and when it shifts with larger tires. Also read a couple of posts that said larger tires changed when the truck shifted in and out of O/D. You got 285's on yours and haven't had any problems? Any rubbing?

I've got over 100,000 miles with 35s and haven't had any issues with weird downshifting or anything. Like I previously stated, the transmission shifts and downshifts according to the engine RPM and throttle position, so it will definately be shifting and downshifting at different MPH with bigger tires, but will still do it at the same RPM throttle position points, so there's no effect to the engine or transmission.

Don't care about the speedo. But wasn't sure how much the tranny was linked to it. I just dont want to srew up the tranny. Its been great so far and I hate to do something that I will regret. Spoke to a couple of mechanics that said I might change the way it shifts and when it shifts with larger tires. Also read a couple of posts that said larger tires changed when the truck shifted in and out of O/D. You got 285's on yours and haven't had any problems? Any rubbing?


Yeah the tranny will be fine. I've had no problems at all with rubbing. Of course, I still have the stock wheels too. I bought the truck with 245's on it that were fairly new and it didn't take long to make junk out of them. I guess the torque just eats those small tires up. I went up two sizes and I don't have that problem now unless I just mash it all the time.