I have a 1990 f150 with a 302 and 4 speed automatic 4x4. It has 355's with 35 inch tall tires. I tow my sand rail with it. 62 mph is 2000 rpm in drive. It will not pull overdrive with these gears and tires. I would like to be able to use overdrive so the converter will lock up but I am not sure what the proper gear ratio is to do so. I would love to pull about 2500 rpm in overdrive at 65mph. If any of you can help I would appreciate it.

Thanks

 

I'd put 4:56's in it. It's an expensive job, though. You'll likely need new carriers, and you'll probably want L/S in the rear.

 

x2
If you want 25% more rpm you want a 25% shorter gear... calcualtes to 4.44, no one makes that so 4.56 is what you're looking at.