Hot Rods Rear help

Need help want to change 4:11 gears out of my 1939 Chevy pickup , I seen a ring pinion gear set for 1940- 1950 truck that would make it 3:55s . They say it would reduce my rpm 20%. Wondering if it would work on my 39 and if not why? Thanks DBF

 

What engine are you running?

 

What is the diameter of your rear tires?

 

Gear ratios are a mathematical constant. Do a google search for gear ratio /rpm/ mph and you will find charts that you can plug in the variables and answer your questions. For example, 65 mph, 30" tall tire, 4.11 rear equals 3000 rpm. Same constants with a 3.30 rear equals 2400 rpm. So their claim of 20% is exaggerated with the 3.55.

I don't have a Hollanders exchange manual and I don't have personal knowledge of 1939/40 Chev rearends, but if they list it for 40-50, something must be different.

You didn't say, we presume it is a stock pickup. Always take the time to list ALL the pertinent information. For all we know it could be a 396, Th400 with a 9" rear.

 

Original 216

 

The truck is all original, like to keep it that way just looking for another 10mph highway speed

 

Splash oiled Chevy 6's do not like to be under-revved (or over-revved).

I have seen that pound the bearings right out of them.

If you are changing out the rear end gears, then you are not keeping it all original. What you are asking for is something that this vehicle was not designed to do. If you manage to track down a set of gears, pay to have them installed properly, and get that 10mph, you will have spent a small fortune on what is likely to be a short lived engine.