Hi guys, I've been searching and lurking for awhile, but have not found any information on transmission shift issues with running larger tires.


Recently upgraded to 285/70 R17 from 265/70 R17 on my 2012 Suburban. Thought the slight increase (only 1.5 inches in diameter) would not have a major effect on how the truck ran. To my surprise, the fuel economy dramatically decreased, but I think that is due to the shift pattern of the transmission. The odometer is only 2 miles off from 60- 70mph. Dealer will not perform recalibration due to the tire size is not supported by Chevy. How have others worked thru this issue?


Anyone using Hypertech type product?


Thanks for the info. And sorry if it's out there already

 

I am running 265's on my suburban 2500. The only tire offered on my vehicle was the
245's. my dealer tried to recalibrate my speedo but there are no Paramiters for the larger tire since it was never offered. I am running an Edge CS and the speed shown matches the speed shown on my Kenwood Nav. radio when i'm using the GPS /nav. mode Mine shifts fine and the slightly larger tires have no effect on the 6.0 motor

 

Your speedometer reading will be approximately 4 percent less than your actual speed with a larger diameter tire.

Not sure how this affects your transmission unless the computer senses the difference between the RPM and the expected speed.

 

Not sure how the 6 speed reacts, but in a 4 speed the shift points feel fine. Acceleration from a stop will be bit slower and towing may feel a bit more sluggish with the bigger tires. 1 1/2" doesn't sound like much but it's a noticeable difference in my opinion.


Fuel economy - I've put larger tires on a few vehicles, and in each case the mpg drop was significant. You have a larger footprint, more rotating mass, and takes more energy to get going from a stop or increase speed. Again, no experience doing this on a 6-speed, but I would have predicted a 10-15% decrease in mileage with that tire change. Curious what others here will say that have done it on a 2009-up with the 6 speed.

 

Thanks guys, everyone is on target with info. It appears that even the slight change in tire diameter has an effect on the 6sp transmission shift timing. Thus, leading to more throttle input on my part and poor fuel economy. I was expecting about a 10% decrease with the new tires but it feels (unscientifically) like 50%.


I tested using the towing button, which I think changes the shift timing by holding the rpm's a little higher before shifting and the seemed to run more like before the tires. Not a solution, just a test.


So, spoke with the guys at Hypertech and they have a programmer to reprogram tire size which they say may be enough to fix the issue, but it also allows you to customize the shift timing of the 6 sp if it needs more help.


Hopefully this will help. I'll keep you posted after I reprogram.

 

May want to check out Blackbear Performance before you invest in a programmer.

For actual experience, read the first post on this link.

 

Zink, I haven't decided if I'll get a tune for mine, but if I do it will be Black Bear, too much positive feedback on their work to go with anything else.


On economy - I agree with you that programming the correct tire size will fix the shift points. I don't agree that the altered shift points are having that much to do with the decreased mileage. Check out a few full tanks to see what you are getting, but in my experience it's the tires, not the shift points.

 

Guys excellent info. Black Bear looks awesome, but limited products for my 6sp auto tranny. The Autocal programmer, or a personal tune is their only option. Anyone have first hand experience with the Black Bear programmer?


Dave01, I'll track mileage over multiple tanks to get a more scientific idea of mpg. I hope the decrease in mpg is just my imagination cause if it's not, I won't be able to afford running these tires.


Thanks again, you've been a great help.

 

Guys once again thanks for the advice. Added the Superchip cortex to reset tire size and transmission was back to normal. As a bonus the Superchip can also smooth out the shifts which is even better now. MPG seems to be better than before. Should know after a couple tanks.


Only thing was after an hour driving Stabilitrak & traction control light came on an and both shut off. After stopping and restarting engine the codes went away and both systems have worked properly for the last few days. I think this might just be a sensor connection issue, but after reading all the other trouble with the Stabilitrak warning light has me worried I messed something up. May need more advice latter.

 

heres a thought from a tire guys perspective. although you didnt increase size to much(just right imo actually) but you increased a hole lot in Weight. weight at the very outside of the wheel where the vehicle has to work the hardest to turn the lightweight 4 ply tires it came with. even just replacing the 265-70-17s upgraded to a 10 ply(something that i do every day out here) will sap your economy. the shift points adjusted could help with that a little, but i think that maybe just relearning how it drives with the new meats will bring you close to norm. aah...that said your highway driving adjusted like with a gps cruising long range should be close. going to bigger wheels with big low profile mudders will do the same thing too just in bigger increments. i have seen some people with 34s or 35s will actually change gear ratios with their bigger tires And have the shift points reprogrammed and stay really close to their mpgs under 'regular' driving.

anyhoo. my2cents lol.

edit. see above lol. dave01 said it, i missed it . sorry for doubling up on info haha