Rear Ends

[Theredsquare]

I have a 1985 cus delux k20. 4 speed (or 3 speed with granny low) and I was just curious about my rear end. I have a 70 ford 3/4 ton with 4.10s and it and my chevy are polar opposites. I was just wondering about what it might be. It tops out about 85-90. It's pretty bad with a trailer which sucks because I haul pretty often, I've been tempted about a swap but wanted some ideas first

 

[bucket]

3.73 and 4.10 are the most common ratios in a K20.

 

[crazy4offroad]

Cam profile can be a limiting factor of your top end as well. Or if the engine is a complete used-up dishrag.

 

[yevgenievich]

My old 88 with sm465 was at 3k rpm by 65 mph. Figured it had 4.56 gears or similar. Figure out what gears you need and put that in the rear

 

[Keith Seymore]

Need more detail*: it depends on engine type and GVW, and Federal emissions vs California.

Base ratio for K20 with gas engine is 3.23 with a 3.42 optional; diesels go 3.73 or 4.10.

You can check your SPID label (if you still have it). If it is base axle it will show RPO GQ1 (regardless of the actual ratio); if it is an optional ratio it will show the specific RPO and call it out by name.

That's assuming it's never been changed. Best thing to do is pull the cover and count teeth to be sure. Plus you can change the fluid while you are at it since it probably needs it.

K

https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1985-Chevrolet-Truck.pdf

* I see your profile says '85 C20 w 350 but your post says K20. Is this a different truck? - because the answer will be different.