GEAR RATIO CHARTS

GM Rat · July 8, 2022

I'm looking for a gear ratio and tire chart for the GM 6 speed transmission. I have a 3.42 rear end and I'm wanting to put 33 tires on. I can't seem to find a chart to reflect the 6-speed transmission.

swathdiver · July 8, 2022

On 7/8/2022 at 11:07 AM, GM Rat said:

I'm looking for a gear ratio and tire chart for the GM 6 speed transmission. I have a 3.42 rear end and I'm wanting to put 33 tires on. I can't seem to find a chart to reflect the 6-speed transmission.

I made my own with excel. Attached is the file or you can ask me what you're wanting to accomplish and I can run the numbers.

00 Towing Calculator and GMT900 SUV Driveline Calculators.xlsx

Edited July 14, 2022 by swathdiver
Updated File 7/14/2022

GM Rat · July 8, 2022

Nice chart you have there. I'm putting on 275/70/R18 tires, I have the 6 speed trans with a 3.42 rear end. My stock tires were 265/65/R18. I'm looking for a chart that will show me the rpm's with different gear ratio options. I plan on getting a tuner as well. I never tow anything. I'm just a city and highway driver. I just want to make sure I don't need a different gear ratio. I don't want to put excessive stress on my trans or engine.

swathdiver · July 9, 2022

6 hours ago, GM Rat said:

Nice chart you have there. I'm putting on 275/70/R18 tires, I have the 6 speed trans with a 3.42 rear end. My stock tires were 265/65/R18. I'm looking for a chart that will show me the rpm's with different gear ratio options. I plan on getting a tuner as well. I never tow anything. I'm just a city and highway driver. I just want to make sure I don't need a different gear ratio. I don't want to put excessive stress on my trans or engine.

Well, you can change all that in the calculator.

On the left under "Tires & Wheels", you can change the tire size and the calculator will adjust. You can even track RPMs as the tires age and wear down by changing the tread depth.

Under "Tires & Wheels" there is the transmission and transfer case calculators. The green field is where you enter the axle ratio and this will update the calculator to show your rpms in every gear at every speed.

Right now your stock tires and 3.42 gears are turning 1,751 RPMs @ 70 MPH.

Swap to the desired tires and you'll be turning 1,667 RPMs @ 70 MPH.

Using the new tires with 3.73 gears and you'll be turning 1,818 RPMs @ 70 MPH.

You will not be "straining" the drivetrain with the larger tires. You don't NEED to change the gears.

The previous generation to yours, mine, ran 3.08s through 3.73s behind the 6L80. The 3.73s being reserved for the max towing package (NHT) in the pickups which some of the earlier models of your generation ran also with the 6L80.

Edited July 9, 2022 by swathdiver
grammar

CamGTP · July 9, 2022

Just don't buy LT rated tires, those are just heavier and not needed on a half ton truck.

Stock gear ratio is more than fine. Pretty sure I commented that in a previous thread.

Gangly · July 12, 2022

On 7/8/2022 at 9:37 PM, swathdiver said:

Well, you can change all that in the calculator.

On the left under "Tires & Wheels", you can change the tire size and the calculator will adjust. You can even track RPMs as the tires age and wear down by changing the tread depth.

Under "Tires & Wheels" there is the transmission and transfer case calculators. The green field is where you enter the axle ratio and this will update the calculator to show your rpms in every gear at every speed.

Right now your stock tires and 3.42 gears are turning 1,751 RPMs @ 70 MPH.

Swap to the desired tires and you'll be turning 1,667 RPMs @ 70 MPH.

Using the new tires with 3.73 gears and you'll be turning 1,818 RPMs @ 70 MPH.

You will not be "straining" the drivetrain with the larger tires. You don't NEED to change the gears.

The previous generation to yours, mine, ran 3.08s through 3.73s behind the 6L80. The 3.73s being reserved for the max towing package (NHT) in the pickups which some of the earlier models of your generation ran also with the 6L80.

swathdiver...GREAT CHART/TABLE!!!

I noticed you used a .9696 multiplier for rolling tire dimensions, but how'd you come up with that value?

Edited July 12, 2022 by Gangly

Grumpy Bear · July 13, 2022

7 hours ago, Gangly said:

swathdiver...GREAT CHART/TABLE!!!

I noticed you used a .9696 multiplier for rolling tire dimensions, but how'd you come up with that value?

I use the actual measured roll out distance. Paint dot

But I'd like to know too....

swathdiver · July 13, 2022

11 hours ago, Gangly said:

swathdiver...GREAT CHART/TABLE!!!

I noticed you used a .9696 multiplier for rolling tire dimensions, but how'd you come up with that value?

Came across that nugget from a tire engineer's website, it's their industry standard so I adopted it.

I tested it with my stock size tires and the 35s on the Sierra and it was perfect.

Thanks for the compliment! I rarely get feedback and wonder if folks thought I was nuts!

Edited July 13, 2022 by swathdiver

KARNUT · July 13, 2022

Most new vehicles are dancing through gears anyway at the slightest incline. Transmissions are so smooth, engines are so quiet people don’t notice. You’ll probably effect the cylinder cut off. I still believe that pretty useless anyway. So there’s that.

Gangly · July 13, 2022

4 hours ago, swathdiver said:

Came across that nugget from a tire engineer's website, it's their industry standard so I adopted it.

I tested it with my stock size tires and the 35s on the Sierra and it was perfect.

Thanks for the compliment! I rarely get feedback and wonder if folks thought I was nuts!

I have a spread sheet I created that is very similar to yours, but I was using a .015-.025 multiplier for rolling variance that I picked up from another site a long time ago, but even they didn't reference where they got that value from. Thanks for the input, I appreciate it.

Gangly · July 13, 2022

3 minutes ago, KARNUT said:

Most new vehicles are dancing through gears anyway at the slightest incline. Transmissions are so smooth, engines are so quiet people don’t notice. You’ll probably effect the cylinder cut off. I still believe that pretty useless anyway. So there’s that.

Yeah, add torque converter lockup and unlock to cylinder deactivation, designed slip values, and convoluted shift map tables and you've created a conundrum of sorts when trying to figure out how to extract accurate and meaningful data in very specific circumstances that modern transmissions seem to fly though.

KARNUT · July 13, 2022

7 minutes ago, Gangly said:

Yeah, add torque converter lockup and unlock to cylinder deactivation, designed slip values, and convoluted shift map tables and you've created a conundrum of sorts when trying to figure out how to extract accurate and meaningful data in very specific circumstances that modern transmissions seem to fly though.

I was with them up to six speeds. When the eighth speed came out car and driver 0-60 test was slower. I can see more gears for towing to a point. Most 1/2 tons are no more than family haulers. I haven’t driven one. I imagine there’s lots of skip shifting, it only makes sense. Reminds me when I drove big trucks. I only hit all the gears loaded. I’ll put 8-10 speeds next to cylinder deactivation. Useless.

shakenfake · July 13, 2022

The 10 speeds are monsters in the Ford trucks. In the GM meh not as good.

customboss · July 13, 2022

On 7/8/2022 at 8:44 PM, CamGTP said:

Just don't buy LT rated tires, those are just heavier and not needed on a half ton truck.

Stock gear ratio is more than fine. Pretty sure I commented that in a previous thread.

Cam, agree with that for 90% of suburban pickup drivers. Out here in rough country you will puncture or rip those pretty fast.

Sad for me because I don't tow but even C rated tires don't have enough ply rating anymore or I am down in the middle of a rocky mountain road asking my wife to hold me up to change the tire or air it up......

swathdiver · July 14, 2022

I updated and replaced the file above. Makes it a little more user friendly and informative and deletes some useless and incomplete information.

Update, tweaked it again just now. It's never been peer reviewed and it wasn't built by a committee or team of engineers, just me. I fix it and modify it along the way.

Edited July 14, 2022 by swathdiver