My 67 truck came originally with the 352 engine and (code C) 3-speed transmission, and rear axle (code 17) 325 gear. The guy i bought the truck from removed the 3 speed trans and installed a C6 automatic, and i now have installed 18 inch rear wheels and tires on it. Now at 65 mph its turning more RPMs than i like on the interstate. I thought some of you might be running this same ( C6 trans,18"wheel) setup and could tell me what rear gear you might be running to get decent rpms at 65-75 mph on the interstates ? Also on these 325 gear, what spline axles were used ? Thanks for any information you might provide.

 

B7TZ-4234-A (replaced B7C-4234-A) .. 9" Rear Axle Shaft with 28 splines / Obsolete

Applications: 1957/67 F100.

 

not sure I'm understanding what you are saying in your reply ?

 

1957/67 F100's w/a 9" rear axle have 28 spline axle shafts.

The axle shafts are the same right & left for these years, why I listed the part number.

 

got you ! Thanks

 

Google a speed calculator, there are hundreds of them. Plug in the info and you can see what the rps are. 18 inch wheels don'r mean much, you need tire diameter and if you have some low-pro car tires on it or have some Pickup 35s is going to make a world of difference.

 

Yeah, wheel diameter doesn't mean jack....

3.25 gears are pretty good. Get taller tires or swap for some 2.75 gears.

 

What size tires are you running? My dent is lowered 5.5" in the back and I can still run 29" tall tires

 

my back tires are 29" tall .

 

Get taller tires, swap to 2.75 gears, or put a gear-vendors overdrive on the back of the C6.

 

So you are cruising 70 about 2600 rpms. The 2.75s would put that about 2200. But you can add a couple hundred rpms with the C6. IMO, that transmission only belongs behind a big block because it robs so much power. The best answer is to instll some sort of overdrive.

 

Ding ding. Nothing is going to make as great of a change (improvement) than having an overdrive. Going with high gears (lower numerically) will give you less engine RPMs and gas consumption at highway speeds but, the trade off is the truck will be less responsive to acceleration and there will be more difficulty pulling a trailer/hauling a heavy load.

Having lower gears (higher numerically) gives better acceleration response and greater ability with ease to pull/haul a heavy load. The downside is higher engine RPMs at highway speed and greater gas consumption.

With an overdrive, you can keep your rear end gear ratio and accomplish both scenarios.

 

I want be pulling anything or even hauling anything.Was just looking to lower the RPMs for hwy cruising. This is kinda a cheap fix desent looking (shop truck sorta) and I really didn't want to change the transmission.Thanks

 

BIGGER DIAMETER TIRES IS THE CHEAPEST FIX

 

Agreed.

Depending on what you want/ like/ can live with, I've seen 35x10.50 tires for 17" wheels. Those would do the job haha

But more realistically speaking, 31s, 32s or 33s would probably be a better fit.