gear ratio
 

i am going to change engines from a 355 to a 421,i have a choise of rear gears,456 or 486,car weighs 2780,power glide trans 14-32 tires.

I'm assuming you're running 1/8 mi only. I think the 4.86 would be quicker, but if you're also running 1/4 mi, you might be twisting the 421 a little tight.

JMO

Zip.

I agree with zipper. the 4.56 would be a gear that you could run both 1/8 and 1/4 without issue. However, if your mostly 1/8 mile go with the 4.86.

RPM formula
 

this is an easy formula that seems to work....

MPH x Gear Ratio x 336 divided by Tire Diameter(inches)

say 100 mph, 4.86 gears, 30" tires.....= 5443 RPM

if you want to get fancy, you can add an inch or whatever for Tire growth, and add 5-10% for convertor slippage.....anyways, it works!