I got a '05 2500HD, bought it new, only has 24K miles, took the 245's off immediately and put 265's on it. Never really cared about mileage, then i put 285's on it for looks of course has been that way for few years. I really think the 285's are killing the power. I bought the hd because i wanted the 4.10's cause I figure the 4.10's and 285's would get me back closer to a 3.73 ratio and I didn't want to put 285's with 3.73's cause I thought it would be too high geared then. I had noticed that you have to give it a lot of gas to get it to move, and then I read that the 4l80 has a high first gear ratio. Another guy I know had to 285's on the same type of truck, and went back to 265's and said his mileage went up a couple mpg's. What are anybody elses opinions or ideas? I did program my tire size with a tech II, but have also heard that doesnt correct the speedo? true or false Thanks for any info

The tire size corrected by the Tech 2 should make the speedo accurate. You can check with a GPS also. Going back to 265's will help in the gas mileage. I went from 265/70/16 to 265/75/16 on my 4.3 gasser and my mpg's went down about 1 mpg, and I noticed that I had to use more of the throttle to get moving. The difference in tire height is only an inch.

The 285's are alot bigger than the 245's, or even the 265's, so going to even the 265's will lower the rolling resistance...

I love how my truck works with 295's and a rancho 4" lift. I corrected the speedo with HP Tuners and just by changing the tire size it changed shift points and a lot of other things too. I can get about 14.5-15 mpg's on the highway and overall 12-13 mpgs hand calculated. We also took torque management down to 25% of the stock setting and changed the tranny pressures to the same pressures in the tow haul mode. Its like a whole different animal. I would recommend this set up to anyone.

I got a '05 2500HD, bought it new, only has 24K miles, took the 245's off immediately and put 265's on it. Never really cared about mileage, then i put 285's on it for looks of course has been that way for few years. I really think the 285's are killing the power. I bought the hd because i wanted the 4.10's cause I figure the 4.10's and 285's would get me back closer to a 3.73 ratio and I didn't want to put 285's with 3.73's cause I thought it would be too high geared then. I had noticed that you have to give it a lot of gas to get it to move, and then I read that the 4l80 has a high first gear ratio. Another guy I know had to 285's on the same type of truck, and went back to 265's and said his mileage went up a couple mpg's. What are anybody elses opinions or ideas? I did program my tire size with a tech II, but have also heard that doesnt correct the speedo? true or false Thanks for any info


Yeah the downfall of the 4L80E is the very high first gear. Tough to get things moving. I have 265/75/16's on my 2500HD now and it did zap a little power away as well as mileage. But it looks way better and gives me a little more ground clearance for going out into pastures and stuff. I used to have a 2000 1500 5.3/4.10 that I put 285's on. It really hurt the mileage and the thing rode like crap. I vowed I would never do that again. I think the thing with the 285's isn't so much the height, but the greater rolling resistance. More surface area on the ground gives better traction, but takes more power to move. I would suggest a mild tune to wake the 6.0 up. My blackbear tune basically returned the power I lost switching to the larger 265's and maybe a little extra. Mileage didn't increase with the tune, but I romp on it alot more too. Taking some torque management out of the tranny coupled with some timing adjustments to the engine makes a big difference with these 6.0/4L80E's. That's my opinion anyways.

I got a friend that works at the dealer as the and he told me you couldnt change the tire size in the Tech II to 285. He said for the '06 the only option it showed was 245. Is there a way you can do it and he just doesnt know how to work the Tech II or do i just need to wait till i get the truck reprogrammed

Thats not the case on that body style. When you get the truck info into the tech II go to chassis,then special fuctions, then tire size calibration, and follow along. There is probably 20 tire sizes and the 285/75r16 is almost at the end. I think 285 is the largest size in the tech II.

OK thanks

OK so I talked to the dealer today and they told me they had to call in to GM and get a program written for my truck. I told the guy it wasnt needed for the '06's and you can do it with the tech II but he's still thinking you need to have the program written. I will have to get ahold of the guy I know there and see if he can bring one home or something