I want to run a different rear gear in a 9" rear axle. Right now it has 456 and even though I drove it mostly around town I'd like something a little better for short trips on the highway. I am leaning towards 411's but also wonder if there is much difference between 411 and something like 389? Is it that big of a step between the two? I am going to be running them in a '68 Fastback with a 4 spd toploader and a 302.

 

4.56 would be a major pain on the highway. The 289HP-equipped cars came with a 3.50, which would be great on the highway. 3.70 or 3.89 would be fine if you did very short highway drives.

 

Easy way to tell what MPH you will be running with a certain gear is an easy math thing.

(gear ratio x 336) / (rpm x tire diameter)

eg. Assuming 26" rear tires and a 2000 rpm cruise speed.
3.89 x 336 = 1307
2000 x 26 = 52000

divide 52000 by 1307 and you get 39.78mph

4.11 x 336 = 1380 divided into 52000 gives you 37.68 mph.
3.50 x 336 = 1176 for 44.21 mph

Moving the values around in the equation allows you to find different things. I use this one because I usually have a target RPM and want to know what mph I will be traveling at that RPM.

What part of Michigan are you from? I live in the UP near Marquette. If you are up here we could work something out to get things done on your Mustang.

 

It was a pain... fun on the on-ramps but that fun didn't last too long. Thank you for your reply, that helps.

 

Thanks for the equation. I like working with exact numbers even though there always seem to be variables along the way. I live in the Grand Rapids area so that could be a bit of a drive. I'm learning as I go. I get offers around here to help but it's funny how everyone wants to be the one to put it together and nobody wants to drill out spot welds!

 

A few calculators I have kept you might like to play with..

Engine RPM Calc
Tire Calc

 

I would do 3.73s if I planned on doing any highway driving. But that's just me lol

 

Thanks for the calc links Adrenolin

 

For more fun with math, you can figure cubic inch displacement with this one.

bore x bore x stroke x .7854 x number of cylinders = ci

I keep them stored in my head because I dont always have a computer handy, but I can usually find a calculator or a piece of paper..