Differential highway geats

[jeffbcn]

47 thriftmaster 1962 235 with Dragon Fire ignition. I believe it still has stock 4 speed floor shift and differential.
My question is what gear ratio in the differential would alow it to run at 65 or so on the highway? Right now it runs great and would probably go over 80 but I don't want to over Rev it. At 65 it runs 3,000 rpm. It gets about 10 mpg.
What RPM is recommended for cruising and what gears would I need to get it there?
Any comments would be appreciated.

[mongocanfly]

what rear gears do you have now? If you have the trans 4th gear ratio, rear end ratio and tire size you can do the math and it will tell you what rear ratio you need for the rpm you want to run.

[jeffbcn]

I think 411. What is a decent highway rpm for a 235?

[mr48chev]

Most guys go to the 3.55 gears. Several vendors I checked sell them usually around 450.00 for the ring and pinion. Here is a set on Ebay for 389 but you might want to do some more searching. http://www.ebay.com/itm/1940-1941-19...25.m3641.l6368

by checking one of the tire/gear/rpm calculators online you can figure out what your rpm drop should be at 65.

This one that Randy's ring and pinion has is easy to use and even gives the rpm for different trans ratios. You want the manual ratio which is 1 to 1
https://www.ringpinion.com/calculators/Calc_RPM.aspx

[1project2many]

I found this article very interesting:

http://vcca.org/forum/ubbthreads.php...rear_gear_conv

[jeffbcn]

Interesting article but that was for a 37. Mines a 47, probably similar to the 51 mentioned.

[curmudgeon]

"Best" rpm for a stovebolt would be where your particular engine is putting out maximum torque, based on cam grind, displacement etc, (generally around 2500 rpm). Stock rear is 4.11, the 3.55 set is the only aftermarket replacement for the torque tube.

http://www.stovebolt.com/ubbthreads/...&Number=340589

The I-15's around Vegas are fast freeways, at "only" 65 mph I would be a moving roadblock. Many of us replace the torque tube with an open differential, I'm running a 3.08 rear which allows me to reasonably keep up with Calif freeway speeds. Of course you have a really nice original truck which you probably want to keep stock, so 3.55 gears are your best bet.

[OrrieG]

Here is the dyno information for a 235. They are not a high rpm engine, HP peaks at 3800, torque at 2000.

[1project2many]

Originally Posted by jeffbcn

Interesting article but that was for a 37. Mines a 47, probably similar to the 51 mentioned.

True, but it provides some insight into another route that might be possible.

[jeffbcn]

Thank you all for the valuable information. It seems a 3.55 is best and drive it on the highway from about 1800 to 2000 rpm. Right now at 45 it's about 1900 rpm. Hopefully a 3.55 will take me up to 60 to 65.