I am planning on changing my rear end gears which are currently 3.92. I have the flathead and a fordomatic transmission. I am most wanting to cruise on the highway at 65-70. Right now my top speed is about 45-50. Any advice on which gears?

 

Ypu have come to the right place. In fact there is a new thread on this subject, and one respose even has a chart with RPM vs rear ratio based on 28" diameter tires. Here a link:

https://www.ford-trucks.com/forums/1...ar-change.html

 

With the automatic I'd go with a 3:25 or even a 3:00 for the best gas mileage and cruising speed...

 

What shape is the Fordamatic in? It would be a shame to go to the trouble to change rear ends, only to be slapping in an AOD in a year or two, and need the old gears.

 

Yup, if you decide to dump the flathead and Fordomatic the 3.92 gears would go great with an OD transmission but a C4 or C6 would put you back to a 3:25...

 

I plan to keep the flathead and the fordomatic even if they both need to be rebuilt one day...both seem great right now though. I started this thread to be fordomatic specific. I figured most on here have a manual transmission so their gearing needs may be a lot different than mine. I was thinking of going with 3.08. I don't want it to be a complete dog though. Thanks.

 

I suspect it would be a total dog. I ran 3.25's in mine for a week or so (manual trans) and it was just too much. My tires are 28.1" rolling diameter.

I'd highly encourage you to measure your tires' rolling diameter as follows: with the truck on flat level concrete, make a mark at the bottom of the tire and on the concrete aligned with the tire mark. Roll the truck forward or backward until the tire mark is at the bottom again. Make another mark on the ground. Measure between the marks. Divide that measurement by 3.14. That is your rolling diameter. It can be very different from advertised diameter.