Gears

[wdbill]

I have a 1985 K20 suburban. It has a 350 th400 and 4.10 gears. I'm looking to lower the rpm's. It has 33" tires on it. When I'm doing 45 mph the rpm's are at 2000. I called Differential shop they recommended a 3.42 gear set. I found a rpm calculator. And the 3.55 gears should run at 2182 rpms at 60 mph. And the 3.42 at 60 has 2108 rpm's. Is there a big difference in power 3.42 and 3.55?

[kwmech]

3.42 divided by 3.55 yields only a 3.7 percent difference. But with that being said you cannot get a 3.55 ratio anyways for GM. 4.56, 4.10, 3.73, 3.42 are the matching ratios

[wdbill]

I didn't know that. Thanks

[James Lamb]

My '89 Suburban came with the 3.42 diff. It's fine. I have towed with mine - 5-6000 lbs, no issues. I think a 4:10 in these trucks is overkill unless you 4 x 4 frequently doing rock crawling. In which case you could just use 4Lo. I'm sure it will help your rpms/fuel economy.

[kwmech]

Originally Posted by James Lamb

My '89 Suburban came with the 3.42 diff. It's fine. I have towed with mine - 5-6000 lbs, no issues. I think a 4:10 in these trucks is overkill unless you 4 x 4 frequently doing rock crawling. In which case you could just use 4Lo. I'm sure it will help your rpms/fuel economy.

Meh, I've got 4.56 gears in mine with a 4l80e and 285/16 tires, runs sweet down the road and pulling a trailer is a breeze. mine is however a converted to 1 ton. 1/2 ton, stock tires you're correct, drive and enjoy. Occasional towing with the 700r is a little rough on the trans with any amount of substantial weight

[James Lamb]

Agreed RE 700R4. I had one that was built by a transmission shop with the heavier components and it did fine towing. But generally they are just too picky for my tastes, and hard to find anyone who knows how to work on them, at least around here. I had a beefed up 4l60e and aftermarket controller put in mine. Much nicer, plus lock up in 3rd and 4th. I would have gone 4l80e if I frequently do heavy towing.