2013 Tahoe 6 speed transmission overdrive

Tahoe man · April 4, 2021

I put 35s on my 2013 tahoe and it shift fine and everything except when it shifts into overdrive it seems to only be turning 1200 rpms and it will drop down a gear and will go back into overdrive, this did not happen until I put the tires on it.

CamGTP · April 4, 2021

You need to tune the computer for the much larger tires.

It's never going to be in or want to be in the right gear because the speedo is so far off now. The shift pattern and shift speed parameters are based on a 30.87in tire size or ~32in depending on what is your factory tire size ( The stock 265/65/18's in GM programming are 30.87 even though that size is 31.5).

Once the computer is tuned for the much larger tires it will adjust the shift speeds so it won't downshift as much. You can also have it wait to use 6th gear until speeds above 50mph or higher if you so choose on a custom tune with something like HP Tuners.

If even after that it doesn't feel exactly how you want then you'd need to swap gears because if you have the factory 3.42 gears those aren't exactly ideal for 35's. Something like 3.73's would help or really 4.10's would be the best so it doesn't need to downshift at the smallest hill.

Edited April 4, 2021 by CamGTP

Tahoe man · April 4, 2021

10 hours ago, CamGTP said:

You need to tune the computer for the much larger tires.

It's never going to be in or want to be in the right gear because the speedo is so far off now. The shift pattern and shift speed parameters are based on a 30.87in tire size or ~32in depending on what is your factory tire size ( The stock 265/65/18's in GM programming are 30.87 even though that size is 31.5).

Once the computer is tuned for the much larger tires it will adjust the shift speeds so it won't downshift as much. You can also have it wait to use 6th gear until speeds above 50mph or higher if you so choose on a custom tune with something like HP Tuners.

If even after that it doesn't feel exactly how you want then you'd need to swap gears because if you have the factory 3.42 gears those aren't exactly ideal for 35's. Something like 3.73's would help or really 4.10's would be the best so it doesn't need to downshift at the smallest hill.

I carried it to a shop and they hooked the computer up and they said they stopped making it where you can change the tire size in the computer, with the 6 speed automatic so I’m at a lose i don’t know what to do any suggestions?

swathdiver · April 5, 2021

8 hours ago, Tahoe man said:

I carried it to a shop and they hooked the computer up and they said they stopped making it where you can change the tire size in the computer, with the 6 speed automatic so I’m at a lose i don’t know what to do any suggestions?

You need a custom tune that will adjust the transmission for the new tire size AND they can set the speedometer right. On a GMT900 you can not change the tire size, but you can tell the computer what kind of tire you are running and its desired air pressure with a Tech-2. HP Tuners and EFI Live are what is needed to change the speedometer.

We've got 35s on our Sierra and the speedometer is off about 6 mph. Transmission performance is fine because she came with 3.73 gears. Sounds like you have 3.08s!

3.08 gears on a stock Tahoe is about 1550-1600 rpms at 70 mph. With 35s that will bring the rpms down to about 1400 at the same speed.

Tahoe man · April 5, 2021

24 minutes ago, swathdiver said:

You need a custom tune that will adjust the transmission for the new tire size AND they can set the speedometer right. On a GMT900 you can not change the tire size, but you can tell the computer what kind of tire you are running and its desired air pressure with a Tech-2. HP Tuners and EFI Live are what is needed to change the speedometer.

We've got 35s on our Sierra and the speedometer is off about 6 mph. Transmission performance is fine because she came with 3.73 gears. Sounds like you have 3.08s!

3.08 gears on a stock Tahoe is about 1550-1600 rpms at 70 mph. With 35s that will bring the rpms down to about 1400 at the same speed.

So is there something I need to order or carry it to get it tuned somewhere? Or just change the gears?

CamGTP · April 5, 2021

You'd have to get it tuned either way. To correct the transmission and the speedo it needs a tune and if you changed gear ratio's it would need a tune.

If you did it yourself with a tuning software like HP Tuners it will be $400 for the device and credits for tuning. HP Tuners does require that you learn how to use the software yourself.

Other handheld tuners from HyperTech and Diablo I think can tune it as well.

Or you will have to contact a local performance shop that does tuning to get this corrected.

Tahoe man · April 5, 2021

2 minutes ago, CamGTP said:

You'd have to get it tuned either way. To correct the transmission and the speedo it needs a tune and if you changed gear ratio's it would need a tune.

If you did it yourself with a tuning software like HP Tuners it will be $400 for the device and credits for tuning. HP Tuners does require that you learn how to use the software yourself.

Other handheld tuners from HyperTech and Diablo I think can tune it as well.

Or you will have to contact a local performance shop that does tuning to get this corrected.

Thank you for the feedback, I will get it tuned and let you know, thanks so much

Tahoe man · April 8, 2021

I carried it and got it tuned change speedometer but didn’t fix transmission problem, I guess change gearing?

CamGTP · April 8, 2021

Without being able to see the tune I can't really help that much more.

You are still at the mercy of those stock gears though. At cruising speeds it's always going to want to downshift if you give it enough throttle.

Tahoe man · April 9, 2021

1 hour ago, CamGTP said:

Without being able to see the tune I can't really help that much more.

You are still at the mercy of those stock gears though. At cruising speeds it's always going to want to downshift if you give it enough throttle.

It turns 1250rpms at 63 mph it will really aggravate you any recommendations, it downshift if you give it any gas and if your going up a slight incline. I’ve never heard anyone have this problem so I’m really lost

CamGTP · April 9, 2021

Yeah, that sounds about right.

You pretty much turned your 3.42's into roughly 3.12's with the 35's. It lowered your cruising rpm by about 200-250rpm which is a lot.

For example, in 6th gear going 70mph with stock 31's the rpm would be ~1,745 rpm, now with your new tires you are only going to turn ~1,540. So at 63mph that is ~1,570 down to 1,370rpm. The calculator I am using likely does not factor in torque converter lockup which can also drop the rpm more.

Your only options are to drive it in M5 or regear to 3.73's to bring it back to where it was before with 3.42 or go bigger to 4.10's which is the better idea.

It's a very common thing, just overlooked. This self inflicted problem is well known. The less gears the transmission has the more a gear change matters. Vehicles with 8 or 10 speeds suffer way less than 4 speed or 6 speed transmissions. They have far more mechanical advantage with more gears.

Edited April 9, 2021 by CamGTP

Tahoe man · April 9, 2021

37 minutes ago, CamGTP said:

Yeah, that sounds about right.

You pretty much turned your 3.42's into roughly 3.12's with the 35's. It lowered your cruising rpm by about 200-250rpm which is a lot.

For example, in 6th gear going 70mph with stock 31's the rpm would be ~1,745 rpm, now with your new tires you are only going to turn ~1,540. So at 63mph that is ~1,570 down to 1,370rpm. The calculator I am using likely does not factor in torque converter lockup which can also drop the rpm more.

Your only options are to drive it in M5 or regear to 3.73's to bring it back to where it was before with 3.42 or go bigger to 4.10's which is the better idea.

It's a very common thing, just overlooked. This self inflicted problem is well known. The less gears the transmission has the more a gear change matters. Vehicles with 8 or 10 speeds suffer way less than 4 speed or 6 speed transmissions. They have far more mechanical advantage with more gears.

You’ve been so helpful thank you, so is what all would I need ?

CamGTP · April 9, 2021

Front and rear gear sets and install kits with bearings. You'd need other special tools as well if you did it yourself.

Or contact a offroad shop for quotes on different gearing. It's not going to be cheap at all through a shop and maybe $1,500-1,800 if you did it yourself.