Gearing question

[gmartin1215]

Hi,

I need a little help trying to figure gearing for the upgrades I want to do. I have an 09 Tahoe with the 6-speed tranny, and the front/rear diffs have the 3:42 gear ratio. If I want to go to 315/70/17 tires, then what is a recommended gear ratio to consider for the diffs? I know the tranny plays into the overall gear ratio, but that is where I get confused. I would like the gearing to compensate for the larger tires if it will help.

 

[swathdiver]

With stock sized tires you should be turning at about 1698 rpms @ 70 mph. Switching to the 315 tire would mathematically deliver 1561 rpms @ 70 mph.

A 3.73 gear would bring you right back to 1703 rpms at 70 mph. 4.10 gears would raise your 70 mph cruise rpms to 1872.

These are math calculations and do not include converter slippage. How do you plan to put such a large tire in the wheel wells? I was thinking 34x10.50x17 which are 33.5 and still near stock width for mine but they are nearly double the cost.

 

[gmartin1215]

With stock sized tires you should be turning at about 1698 rpms @ 70 mph. Switching to the 315 tire would mathematically deliver 1561 rpms @ 70 mph.

A 3.73 gear would bring you right back to 1703 rpms at 70 mph. 4.10 gears would raise your 70 mph cruise rpms to 1872.

These are math calculations and do not include converter slippage. How do you plan to put such a large tire in the wheel wells? I was thinking 34x10.50x17 which are 33.5 and still near stock width for mine but they are nearly double the cost.

I am putting a 4" lift

 

[swathdiver]

I am putting a 4" lift

Our Sierra has a 6 inch lift with 35x12.50s and it rubs a little on a full turn.

 

[Tozan]

I am running a 4 inch lift with 35's and they only run under compression while turning... I did trim the wheel well area towards the front too. I am running 410 gears. If you need more grunt 410's are a good choice. If you lean a little more towards MPG I think the 373 might be a better choice and you most likely will not be giving up a noticeable amount of power either...

If you want to go bat **** crazy I have a set of 488 gears laying around... You will get 11MPG and no acceleration with them but they do crawl... You will be in 6th gear at 35 mph with the 488's...

 

[02Z71Raven]

I am running a 4 inch lift with 35's and they only run under compression while turning... I did trim the wheel well area towards the front too. I am running 410 gears. If you need more grunt 410's are a good choice. If you lean a little more towards MPG I think the 373 might be a better choice and you most likely will not be giving up a noticeable amount of power either...

If you want to go bat **** crazy I have a set of 488 gears laying around... You will get 11MPG and no acceleration with them but they do crawl... You will be in 6th gear at 35 mph with the 488's...

4.88 sounds awesome. I wish I could put that on a switch on my dash.

 

[swathdiver]

4.88 sounds awesome. I wish I could put that on a switch on my dash.

Well, do you have a 2-speed transfer case? Only problem is your top is about 40 then!

 

[Bill 1960]

I’m running 35” tires and 3.73 which was original in this truck. It’s “OK“ for all around use. Bearing in mind this is a 4 speed so first is tall and acceleration is not great from a standing start. I preferred the feel of 3.73 with the OEM tires myself. On the plus side it makes it easy to drive it gently when off-road which is a consideration with these aluminum front diffs. 3.73 it will get into overdrive and hold it on level ground and moderate grades. A six speed may behave differently; my brother has a Silverado with 35” and 3.73 and says it will not cruise level in 6th.

4.10 would be the closest match to get your RPM close to factory tires & 3.73 gears. That’s what I’d choose for a road truck.

4.56 for a more off-road bias. For most big tire rigs I prefer 4.56 for 35” and 4.88 for 37” for all-around use. I do a lot of dirt.

 

[02Z71Raven]

Well, do you have a 2-speed transfer case? Only problem is your top is about 40 then!

I have 4hi and 4low. Is that 2 speed? Is that 2 different actual gears?
Sorry, but I'm completely ignorant to the way the 4x4 system works. This is my first 4 wheel drive. I grew up under the hoods of old sports cars mostly.

 

[swathdiver]

I have 4hi and 4low. Is that 2 speed? Is that 2 different actual gears?
Sorry, but I'm completely ignorant to the way the 4x4 system works. This is my first 4 wheel drive. I grew up under the hoods of old sports cars mostly.

Yes, if you put the truck into 4LO the motor will turn at higher rpms running at slower speeds. Your first gear would go from a 10.47:1 ratio to 28.47:1 (assuming 3.42 gears).