Upgrading a 1500 4x4 for travel trailer towing

[Marky Dissod]

Well, I started out with 3.42 and cruising at around 1,750RpM and now just over 1,900. So 4.30 would be 300RpM more at the same speed.
Going to hear those extra RpM through the cab. If I were a kid street racing, maybe, but not now.

Use my calculator, it's more accurate!

3.73, 275/65R18, 70MpH, 4L60's 4th = 1915RpM . . . Makes me miss my Caprice wagon with 3.42 & 235/75R15 . . .
4.10, 275/65R18, 70MpH, 4L60's 4th = 2105RpM . . . Soon as I find a 4.10 rear axle and a 4.10 front diff . . .

4.10, 275/65R18, 70MpH, 6L80's 6th = 2015RpM . . . Schidt. Just convinced myself. If I ever get a GMT900 with a 6L80, 4.10 - and I may never tow anything.
By then I'll be old enough to have suffered just enough hearing loss that I either won't mind or won't notice the extra RpMs through the cab.

 

[swathdiver]

3.73, 275/65R18, 70MpH, 4L60's 4th = 1915RpM . . .

Close but no cigar!

 

[Marky Dissod]

Think I'm using unloaded diameter (32.07") vs rolling diameter (31.125").
Also I think you used 0.696 for 4th, vs 0.7.

So, which calculator are you using? I was using

MOTOR - A Rear Axle Calculator

 

[DaveO9]

Another vote here for 3.73 vs 4.10. Same thinking as others - base it on desired RPM for desired speed in 4th gear. Another consideration, the 6L80 has a much lower 1st gear (about 4:1) than the 4L60e (about 3:1). 4L80e was an even higher 1st at about 2.5:1. So older rigs with 4 speeds, low axle ratio was sometimes needed for starts with heavy loads as well as cruising and climbing, but less of a concern with the newer transmissions.

I started with 3.08 gears and just went to 3.42, based on the two considerations above. But I don't plan to tow near as much. (and there are times I wished I went 3.73) I went the 14-bolt route, too, and all is well so far with the rear end. Not so much for the 3.42 used front I got. My other recent posts are about how it is eating a bearing or similar, and I need to replace it.

 

[Marky Dissod]

6L80 has a much lower 1st gear (about 4:1) than the 4L60e (about 3:1). 4L80e was an even higher 1st at about 2.5:1.

10L80: ... 4.69 ... 2.99 ... 2.15 ... 1.77 ... 1.52 ... 1.28 ... 1.000 ... 0.85 ... 0.69 ... 0.64 (7.32 Spread)
8L90E: ... 4.56 ... 2.97 .... 2.08 .... 1.69 . . . . . . . 1.27 ... 1.000 ... 0.85 . . . . . . 0.65 (7.01 Spread)
6L90E: . . . . 4.03 . . . . 2.36 . . . . . . . . . 1.53 . . . . 1.15 . . . . . . . 0.85 . . . . 0.67 (6.01 Spread)
4L60E: . . . . . . . 3.06 . . . . . . . . . . 1.63 . . . . . . . . . . . . 1.000 . . . . . . . 0.69 (4.43 Spread)
4L80E: . . . . . . . . . . . 2.48 . . . . . . . . . . . . 1.48 . . . . . . 1.000 . . . . . 0.75 (3.306 Spread)

Reason why I included 8L90 & 10L90 is, after FIVE DECADES of undergearing to lick CAFE's undercarriage, EVEN GM CONCLUDED:
yea, needs more gear.

4.03 x 4.10 = 16.52 ... 6L80's 1st, times the axle gear GM offered optionally for 2500 & 3500 with 6L90
4.69 x 3.23 = 15.15 ... 10L80's 1st, times 3.23 (nearly every truck / suv with 10L80 has 3.23 axle)
4.03 x 3.73 = 15.03 ... 6L80's 1st, times the axle gear Dave09 suggests (which GM gave to 1500 pickups but NOT suvs with 6L80)

If you care more about unladen highway MpG than towing MpG, 3.73.
If you care more about towing MpG than unladen highway MpG, 4.10.

... I don't plan to tow near as much. (and there are times I wished I went 3.73).

There may be times when bcbickers'll wish 'Dang, should've 4.10'd it' ...