Square body chevy truck gear ratio

[etedeschi72]

Ok, I've got an '85 K20 4x4. Its got a 454 engine, a SM465 4 speed transmission and what I believe (99% sure) are 4:10 gears front and rear with 33x12.50x16.5 tires. I really don't know anything about an RPM range or what a good one woul be but I'm just guessing around the 1800-2000 range. Now, my long term goal is to swap in an LS 5.3 and maybe an NV4500 but neither is happening any time soon. Anyway, I hate the low slow gearing that's in it now and want to change them. I don't use it for off roading or anything like that. It's my daily driver for Spring, Summer and Fall and I do tow trailers occasionally. So my question I guess is, what would be the best all around gearing for what I'm using it for? I'm looking for faster speeds, something that'll go 70-75 maybe on the highway and for occasionally towing. I've heard that 3:55's would be good. Someone please help lol

 

[Grit dog]

I have the same gear ratio truck and it’ll do 70-75 all day long….
You mean those speeds at lower rpm’s?
I’d suggest just spending 10 minutes with any of the online rpm/gear ratio calculators since it’s your truck and your decision and you didn’t provide some of the info needed for anyone to make a recommendation anyways. (Tire size and desired rpm’s at your desired speed). Plus you’ll learn quickly how to answer your own question.

 

[etedeschi72]

Thank you, I'll try it. I didn't realize until you said something that I forgot to put a tire size and RPM'S. I'll edit my post.

 

[K5 Blazer]

Maybe try a bigger tire on it.

 

[75gmck25]

Try one of the rpm calculators and play with gear ratios until you get the highway rpm you want. I have 4.10 gears and stock 31” tires. IIRC, it runs about 2800-3000 rpm at 60 mph, and I usually don’t drive faster than about 65. It’s kind of “busy” at that speed, but it runs smooth and I’m sure it would go a lot faster.

You may have to do some research to figure out what gears you can swap with your axle setup. On some axles the carrier is different for 3.73 and 4.10, so swapping directly to 3.23 or 3.55 is not possible. However, I believe the carrier can also be swapped.

 

[legopnuematic]

An 85 is likely a 14 bolt semi float axle. It could be a full float too, but I’d lean towards SF. If you can get a picture of the rear wheel of it has a hub that stick out proud of the wheel or not that will determine that. Or of the rear differential cover.

Anyways, 3.55 is not an available ratio on a 14b sf, or full float, for a 10 bolt yes, for the 14 bolts you have:
3.42, 3.73, 4.10, 4.56 (and lower).

The 14 bolt semi floats do not use carrier breaks like the 12 bolt truck axles though, so no thick/thin gearsets or carrier swaps required for certain ratio changes. Full floats is 4.10 and down, 4.56 and up for carrier breaks.

The 10 bolt front also do not have carrier breaks (they do but it’s ratios no one will use in a truck, 2.56/2.73)

If you are running 33” tires, 3.73 is a pretty good overall ratio.