so im buying a new rearend kit from curriefor my 67. im converting it to a 9in but the thing is i dont know what kind of gear ratio to go for. i was thinking 3.25:1 or 3.50, its my daily driver but i do go to the track often so im looking for a little more performance too. its not too beefy at the moment. its got a 302 roller from an 89 5.0 (carb not fuel injec) power pulleys, the edelbrok rpm air gap intake, cam and cylinder heads. it has the c4 tranny with shift kit, and kevlar clutches. i know i dont need a 9in for what im running now but my future plans for the car is to blow it. so back to my question, what gear ratio should i go for ?

 

whats in it now?

I would without doubt go to a detroit true trac over a clutch type limted slip/ posi

as far as gearing...I have 3.5's which are brilliant all rounders but i do a fair bit on open road so another gear would be nice.

 

im runing 4.11 gears in my 9inch with a air locker i drive it everyday my rpms are a little higher then i like on the highway but having a aod helps it out some.

 

If its your daily driver on a C4 I would avoid 3.50s (if it ever gets above 70mph for any given time)

3.25 would be probably be okay

I have 3.00 and turn 3200 rpm at 80mph with 25" diameter on rear tire

I TOTALLY second the trutrac its awesome.

Gun

 

What exact camshaft did you choose? Do you like to autocross? Do you like long distance freeway driving? How nutty do you want to get with the blown motor? Are you thinking of a small centrifugal blower, or some monster screw type blower? Do you have a rough idea of your final HP and torque curves?

Here's what got me to choose 3.50's
I like to autocross, and I enjoy nothing better than to launch off the line like a jet on an aircraft carrier.
I will do very little freeway driving, never over100 miles.
I've set my rev limiter at 6500.
My max hp is at about 6250 rpm.
My max torque is at about 3600 rpm

I don't think you could go wrong with either, but I would error on whatever side you do most of your driving, Freeway = 3.25's, 1/8...1/4mi, autocross and terrorizing the town = 3.50's

The Detroit true-trac torque biasing differential is awesome, I can't think of a street application where another style of differential would be better.

Peter