1977 C2500 Transmission and rear diff help!

[cottonme173]

Hello! I have a '77 GMC C2500 that is geared to pull freight trains and rip large things from the earth.....which is great, but I don't need to do any of those things :-)

I have, the stock 4 speed manual transmission L-1-2-3 and 4:11 rear gear ratio.

At 60 MPH I am pushing around 2800 RPM and I think the max is around 3500, either way its screaming. Max speed is about 70 :-( I am looking to use the truck as an occasional driver/project truck and I am wondering what to do to increase max speed and not be screaming RPM's doing so.

I love the manual (dont want automatic) and I am looking for the cheapest route initially.

Do I just swap out the rear end for something mid 3's or do I need a different transmission? Can you re-gear the current transmission? Its a big heavy truck so I know it needs a semi heavy duty tranny....

Any help or insight would be great!

[MalibuSSwagon]

Your cheapest route would be to re-gear to 3.73 or better 3.42. You'll have to determine if you have a 14 full floater or semi-floater.

NV4500 5 speed is a good option as well, that will set you back about 1000-1500 or more for a completed swap.

[crashz]

Transmissions can be re-geared, but that's by far the most expensive option as you'd probably have a custom transmission built for you.

Your best options are:

1) Swap axle gears: Cheapest and probably the simplest, especialy if you have a shop do the work. But you'd lose some of the pulling power. You will also only gain a moderate amount in top speed and mileage. It would leave your classic truck mostly stock, and require absolutley no modifications. If the truck is in pristine condition, this could be a concern.

2) Swap transmissions to a 5 speed with OD: This will require some mods to do, but a good HD 5 speed like a NV4500 can be swapped fairly easily (used in later model GM trucks). It will require a host of minor modifications, but plenty of people have done these swaps and you'll find a bunch of info right here. this would allow you to keep the stump puller gears and decent highway speeds.

3) Aftermarket OD unit: Gear Vendor used to make (maybe they still do) a gear box that would bolt up to the back end of a transmission or T -case. When working, the do a great job and allow gear splitting. They are also know to be fairly rugged. Keeps the stump puller gears, decent road speed. But I've heard these are expensive.

Keep in mind, I only have real hands on expereince with option #1. Someday I'll swap a NV4500 into the 83, but thats a long way off. So take my advice for what its worth.

[cottonme173]

Thank you guys for the info! I was pretty much thinking exactly what you said, so looks like a re gear to start!

Malibu- how do I find out if I have a floater or semi-floater?

Thanks!

[mrolds88]

Full floating axle has a hub that sticks out through the center hole of the wheel when its mounted. It also has 8 bolts that hold it on. A re gearing would certainly work for what you want. Might be just as easy to buy another rear axle and swap them. Also the NV4500 is another great option. It would also give you the best of both worlds. Pulling large objects from the earth on the weekends and overdrive to go to work on the freeway during the week. If you are considering both options together, I wouldn't go any lower than the 3.73's.