My 1977 C2500 has super low gears - ideas?

[Mechanic707]

Hello everyone, I just got a new to me 1977 GMC C2500 V8 5.7L Grande 4 speed standard. (cant forget the glass packs :-)
I have'nt looked at what gears I have yet, but i'll give you a mph ratio. lol
1st gear - 5mph, 2nd gear 20mph, 3rd 35mph, and fourth at 45-50 tops. Super torque.
Can I put some bigger tires on it to get more mph? Are gear ratio's easy to change?


thanks for any input, i'm new on here so be kind brah

[Mechanic707]

I currently have 15" 70 series tires on it. I have 17" 75's would that make a noticable difference in top speed? Or should I just change gear ratio?

[lester622]

Originally Posted by Mechanic707

I currently have 15" 70 series tires on it. I have 17" 75's would that make a noticable difference in top speed? Or should I just change gear ratio?

only way to do it would be change the gear, the tire size only makes so much of a difference on gears and its not that noticable of an amount unless u go from a really short tire to really tall tire.

[James McClure]

707. Lets not go off half cocked. first thing you want to do is find out what gears are in it now and go from there. If you have 4 series gears in it and you change to 3 series your going to need a carrier also and your going to need shims either way. If you haven't done a rear end overhaul before, find someone who has and work with them to do yours so you know how to fix it. It is not easy. If your off by even .001" in backlash or bearing preload it will howl like the devil gone mad. Good luck, jim

[Mechanic707]

I have installed an aussie locker, but I am not familiar & looking forward to learning

If anyone has helpful guides/info/links please post
Also, I know bigger tires will make it go faster but im wondering how much faster

[justcuz]

Look in your glovebox or inside the hood on the fender well to see if you RPO codes are listed. You should be able to tell what your rear axle ratio is from that if the rear axle has not been changed. I would think you would not have more than 4.56 gears in that. You know since you are up in Nor Cal you should see if some one has an old logging truck with an auxilliary transmission.
I have one I am saving to put in a truck someday, it's a 3 speed, over, direct and under. I want to stick it in a 1 ton or bigger flatbed.

[sierra4x4]

Hi there, thats a beautiful truck you have there. You likely have 4.10's in a full floater 14 bolt rear. They are a bear with stock tires ( I have them). A very nice rear is the 3.21 that can be found in some of the camper specials they are awsome for these trucks especially with a big block. Also I think all the carriers are the same for the FF 14 bolt (no 3 or 4 series).

[Dooley]

Originally Posted by Mechanic707

I currently have 15" 70 series tires on it. I have 17" 75's would that make a noticable difference in top speed? Or should I just change gear ratio?

I'm curious as to what 8 lug wheels you have on this truck that would sport 15" tires. A 1977 2500 series GMC came from the factory with 8.75-16 tires and wheels according to this information below. I think that might be a misprint as the 1977 Chevy 20 series came standard with the 8.75-16.5 and I do remember those.

The 1977 C2500 came factory equipped with a standard 4.10 gear. Optional gears included
3.21
3.73 and
4.56