Gear ratio calculator

Giggity231 · June 22, 2022

Hey all. I'm trying to use the gear ratio calculator in the link below. I have a '17 1500, 6l80, 3.42, 33" tires. I pull a pretty heavy camp trailer so about to pull the trigger on a 4.10 gear swap. Trying to find a little more info on the before/after results before pulling the $2200 trigger.

The calculator asks for a transfer case model or gear ratio. Anyone know how to get this info? Not finding anything in the owners manual. I have the electronic controlled 2H/auto/4H/4L case RPO code NQH.

Also can anyone tell me what an 'Underdrive' is? I don't think that applies to these trucks, but I could very well be wrong.

http://www.grimmjeeper.com/gears.html

Any info is truly appreciated

CamGTP · June 22, 2022

You don't really need to be concerned with the hi/low numbers of the transfer case and all that jazz, you aren't using this for crawling trails.

All you need to do it select your transmission, current gear size and your tire size. It will spit out the numbers you should be seeing now. Then just change the gear ratio to 3.73's or 4.10's to show the rpm differences at speed at the bottom.

4.10's will have more low end grunt that's for sure but your highway mpg will suffer because the rpm's will be 300-400rpm higher depending on speed.

swathdiver · June 22, 2022

4 hours ago, Giggity231 said:

Hey all. I'm trying to use the gear ratio calculator in the link below. I have a '17 1500, 6l80, 3.42, 33" tires. I pull a pretty heavy camp trailer so about to pull the trigger on a 4.10 gear swap. Trying to find a little more info on the before/after results before pulling the $2200 trigger.

The calculator asks for a transfer case model or gear ratio. Anyone know how to get this info? Not finding anything in the owners manual. I have the electronic controlled 2H/auto/4H/4L case RPO code NQH.

Also can anyone tell me what an 'Underdrive' is? I don't think that applies to these trucks, but I could very well be wrong.

http://www.grimmjeeper.com/gears.html

Any info is truly appreciated

Do you know the exact tire and size? Is it a flotation tire? I make my own calculators on Excel and can share it with you.

The ratio for the transfer case is 2.68.

Giggity231 · June 23, 2022

6 hours ago, swathdiver said:

Do you know the exact tire and size? Is it a flotation tire? I make my own calculators on Excel and can share it with you.

The ratio for the transfer case is 2.68.

This looks like a pretty awesome tool! As a fellow Excel geek I appreciate how clean and detailed this is. Exact tire size is 275/70R18. I'm not sure what a flotation tire is. They're BFG AT KO2's if that answers your question. Don't know tread depth measurements but they're almost new. Have maybe 1000 miles on them.

Giggity231 · June 23, 2022

7 hours ago, CamGTP said:

You don't really need to be concerned with the hi/low numbers of the transfer case and all that jazz, you aren't using this for crawling trails.

All you need to do it select your transmission, current gear size and your tire size. It will spit out the numbers you should be seeing now. Then just change the gear ratio to 3.73's or 4.10's to show the rpm differences at speed at the bottom.

4.10's will have more low end grunt that's for sure but your highway mpg will suffer because the rpm's will be 300-400rpm higher depending on speed.

Glad to hear that. Most my long range highway driving is pulling, so 300-400 rpm higher would be acceptable.

swathdiver · June 23, 2022

23 minutes ago, Giggity231 said:

This looks like a pretty awesome tool! As a fellow Excel geek I appreciate how clean and detailed this is. Exact tire size is 275/70R18. I'm not sure what a flotation tire is. They're BFG AT KO2's if that answers your question. Don't know tread depth measurements but they're almost new. Have maybe 1000 miles on them.

Thank you, it's a labor of love!

KO2s have 15/32nds tread depth in most cases. For our purposes it does not matter. Flotation tires are the tires known as 30x9.5x15 or 35x12.5x18, etc. People also call them pizza cutters.

That's the size (275-70-18) I was going to put on my 2009 Yukon XL but don't want to level it so they will fit as I tow as well and don't want the nose up high or to use bags to keep the rear end up! That's what my research showed me, always hoping I'm wrong!

Ok so when your truck was stock with stock tires this is what your RPMs looked like mathematically. Add 5-15 rpms for torque converter slip.

Now, this chart below here is what you should be seeing with those 275s (when they were brand new):

This is what your rpms would look like with 3.73 gears:

This is what they would look like with 4.10s:

What do you think? What's the sweet spot of your motor? With stock tires, the Gen IV LSs seem to really like 3.73s and GM obliges them with such in the NHT and 2500 gassers of that generation.

Edited June 23, 2022 by swathdiver

Giggity231 · June 23, 2022

7 hours ago, swathdiver said:

Thank you, it's a labor of love!

KO2s have 15/32nds tread depth in most cases. For our purposes it does not matter. Flotation tires are the tires known as 30x9.5x15 or 35x12.5x18, etc. People also call them pizza cutters.

That's the size (275-70-18) I was going to put on my 2009 Yukon XL but don't want to level it so they will fit as I tow as well and don't want the nose up high or to use bags to keep the rear end up! That's what my research showed me, always hoping I'm wrong!

Ok so when your truck was stock with stock tires this is what your RPMs looked like mathematically. Add 5-15 rpms for torque converter slip.

Now, this chart below here is what you should be seeing with those 275s (when they were brand new):

This is what your rpms would look like with 3.73 gears:1

This is what they would look like with 4.10s:

What do you think? What's the sweet spot of your motor? With stock tires, the Gen IV LSs seem to really like 3.73s and GM obliges them with such in the NHT and 2500 gassers of that generation.

It's nice to see the info like this. If tire/wheel weight wasn't a concern I'd definitely go 3.73. That would bring me just 67 RPM above stock configuration at 70mph. But given the heavier tires 4.10s would probably be best. I knew my stock 20 mph highway driving was dead and gone once I leveled and put the bigger tires on it.

Since NHT trucks come with 3.73s, can you show me this table with stock tires running 3.73s? Wondering how close RPM would be on that vs. the 33's/4.10 on your bottom chart you provided.

swathdiver · June 23, 2022

5 hours ago, Giggity231 said:

It's nice to see the info like this. If tire/wheel weight wasn't a concern I'd definitely go 3.73. That would bring me just 67 RPM above stock configuration at 70mph. But given the heavier tires 4.10s would probably be best. I knew my stock 20 mph highway driving was dead and gone once I leveled and put the bigger tires on it.

Since NHT trucks come with 3.73s, can you show me this table with stock tires running 3.73s? Wondering how close RPM would be on that vs. the 33's/4.10 on your bottom chart you provided.

Mine (daughter's) is lifted on 35s, the tires weigh 81 pounds each, double the original Goodyears. I was figuring to run 4.10s to get it back into the rpms of the 3.73s on stock tires. It gets about 11 in the city and 13 on the highway and is driven aggressively.

Edited June 23, 2022 by swathdiver

Giggity231 · June 23, 2022

2 hours ago, swathdiver said:

Mine (daughter's) is lifted on 35s, the tires weigh 81 pounds each, double the original Goodyears. I was figuring to run 4.10s to get it back into the rpms of the 3.73s on stock tires. It gets about 11 in the city and 13 on the highway and is driven aggressively.

Awesome thanks again for this. Those mpg's are painful...especially now. I have a Jeep that gets around the same but is driven like the slow tank that it is. Needless to say it stays parked until my son gets his learners permit.

swathdiver · June 23, 2022

52 minutes ago, Giggity231 said:

Awesome thanks again for this. Those mpg's are painful...especially now. I have a Jeep that gets around the same but is driven like the slow tank that it is. Needless to say it stays parked until my son gets his learners permit.

I keep telling her to slow down and she says, "Smiles per gallon, Dad!" That truck is refueled every 2-3 days, like an old Navy Destroyer!