Gear Head - know it all

[I Leak Oil]

If you use significantly taller tires than what the factory put on you're best off using one of the on line gear calculators to match what the factory final ratio should be. If not then there is a chance your transmission will be doing a lot of hunting for the right gear. You may not like the constant up\down shifting, the tranny will wear faster and your mileage will suffer, speedometer will be off. Simply having a V8 won't make up for any of this.

 

[CodyY]

Engines run at peak efficiency just before the torque peak.

 

[hillbillyjake]

Mine is almost the same as yours exdept 4 door and a little less weight. Stock 3.42 gears and 35 general ats. I'm hoping to go to 4.56 this summer. Maybe 4.88. I never get any higher than 5th and average 12.5/13 mpg. Dailey driver.

 

[Larry]

When it comes to picking out a gear ratio for a vehicle running a 6L80/90 transmission forget everything you thought you knew about ideal axle gear ratios. 6L80/90s have a very deep first gear ratio of 4:03 (for reference the 4L80E had a 2:48 first gear. The higher the numeric number the "deeper" the gear whereas the lower the numeric number the higher the gear ratio. Think if high gear ratios being ideal for highway cruising). I would not go any lower (high numeric) than 3:73 with any tire shorter than 35”. We briefly had a 2013 Tahoe Z71 with a 6L80 with 3:42 gears. I would never in my mind think 3:42 would considered a deep gear ratio but it sure was in that rig behind that particular transmission. It was a jerky SOB off the line with that super deep 1st gear. In fact, 3:08 might just do you fine behind a 6L80 in low altitude PA. Something like a 4:56 or 4.88 would be horrible behind a 6L80/90 unless your running some ungawdly large tire like a 44 incher.

On the other hand, if you were running a 4L60E or 4L80E, I would say go with 4:10s........ but not with a 6L80/90

 

[CodyY]

Oh, yeah. I'm stupid, didn't register the 6 speed auto. That changes everything.

 

[jimi breeze]

Well Larry, I never really considered the transmission's gears. I understand what your'e saying regarding the first gear being so low. That explains why these trucks are coming from the factory with 3.08's and 3.42's gear ratios.

Also - I am currently running 33" tires (285/70/17) and with the stock 3.08's my transmission shifts into all geRs fine, even 6th gear on the highway. I get approx 15-16 MPG on the highway.

I was was thinking of putting in an air locker and with the convential way of thinking gears, I would put in a lower set.