Posi 3.73?

[tfen2001]

I have a '64 C10 with 292 cubes. The truck tops out at 50-55 mph so I'm looking for help on how to get more top end out of her. I pulled the diff cover to determine the ratio and found it is a 3.73 ratio. At some point I'd like to drop in a 350 with a muncie 4 speed. This maybe a dumb question but upgrading to a 350 4-speed won't change the top end speed, right? So, I'll need to go to a lower ratio in the rear end, right? That being said, I think this may be a 12-bolt posi but I'm not sure how to ID a posi. There are some small coil type springs in the carrier. I haven't been able to find any pics that would show whether this is a posi or not. With the rear wheels off the ground, both wheels spin in the same direction - does that mean anything? Thanks for the help.

[zor]

The rear end ratio determines what rpm you are at for a given speed. So putting a V8 and 4 speed in won't change what rpm you are turning for 55 mph. Taller tires will bring down the RPM's and so will an overdrive trans. I run a t-5 in these trucks. Last one was a 64 with a 230" inline six and 4.11 rear with 31" tall tires. With the T-5 it was pretty comfortable at 70-75 for long stretches and good on the fuel tank as well. I'm building a 65' now and it getting a 292, T-5, and I haven't decided on a rear ratio, but I'm running G78 bias wide whites. So it might be a bit more fun off the line and a little less on top speed.

[tfen2001]

Thanks for the input. Sounds like a great way to go. Still looking for info to determine if this is a posi if anyone has any ideas, let me know. Thanks!

[moses27]

You might try jacking up the truck so both rear wheels are off the ground, rotate one wheel if the other wheel turns in the same direction it most likely is. If it turns in the other direction its prob. open. Hope that helps.

[antivenom]

Springs in the carrier and they spin the same direcion...you have posi for sure.

[tfen2001]

Great, thanks guys. I appreciate the help.