New rear end/differential??

[Coltronist]

Hello my friends,
I have a 63 c10. I just bought a 350 motor that I'm turning into a 383, and a th350 Trans.

I want to try and get this truck to be able to cruise at 80mph and with my little knowledge I thought just putting a new Trans in would work, so I thought of buying a 700r4.

My neighbor from down the street has an apache and was telling me that it's not so much the Trans I need to replace, but the rear end.

Here is the question:

If I replace the rear end, will it make that difference with the th350? Will I be able to cruise at 80 by doing so? And if so, what rear end should I look for that I can use on my truck?

I'm going to school for engines and transmissions starting tomorrow so I don't know too much, just seeking your wisdom

Thanks for the help!
-Colton

[Clyde65]

before you do anything, get under your truck and find out what you have as far as gear ratio. some have tags, some you pop the cover and start looking on the side of the ring gear. it probably needs to have that gear oil changed anyway.


you will have either a 3.08:1, or 3.73:1 and up.

the 3.08 will be fine with the 350.

3.73 and up would really need a 4 speed trans.

Good luck!

[PGSigns]

I'm with Clyde, first find out what you have and we can help with a plan. The best setup for long highway runs would be the 700R4 with a 3.03 gear. Would give you a better ratio for first gear over the 350 and the 3.08 on the highway with the OD should net some good low rpm's and decent fuel mileage.
Jimmy

[65Gregg]

http://www.calculatorpro.com/calculator/rpm-calculator/

[Coltronist]

Thanks so much you guys that helps a lot, I'm gonna find out the gear ratio and then I'll get back to you

[Captainfab]

A 3.08 gear with an OD trans is not always a good idea. You probably wouldn't want to use OD until you were over 60-65 mph. The engine needs to operate in it's 'sweet spot' for max efficiency. Of course there are other factors to consider, such as the engine and how it is built as well as rear tire size. Typically a 3.73 rear gear with a 28-30" tall tire and an OD trans are a good combo.

[66 C10 383]

Rear tire diameter makes a bit of difference too. A taller tire equals more mph at the same rpm.

[LVPhotos]

I posted a rpm chart on build thread. It is the same info everyone needs to start with. My tires are 28 inches tall, not much shorter than stock. 3.73 posi rear at 65 in in 1:1 I am at 2200 to 2400 on the chart. It's is yellow for sweet spot. You can go Higher for mileage or Lower for power. Now it drops with my overdrive to 2000 at 75. My 81 ran 350/350 and 3.08 and 295/50/15 at 3000 rpm at 75to 80.
http://www.adamsdriveshaftoffroad.co...rts-tech-info/