what would be the best rear end gear ratio?

[69projectKAOS]

hi guys, i have a 355 sbc 375hp/402ftlb with a th400 with a shift kit, and i have a gm 10 bolt. i dont know what gears are in it now, but when i was driving it to the body shop i felt that it was kind of winnding out at about 55-60 mph but when i would get on it, it had a little more left. and i was getting a humming sound from the rear end i think but only when i was giving it gas and when i let off the it would go away. what do you think are the best rear end gears for cruising but still have that performance factor as well! thanks-dom

[chicane67]

Without an OD transmission... I would use a 3.42 myself.

With an OD transmission... I would lean toward a 3.73 to 3.90.

[69projectKAOS]

now what is the acceleration and crusing like with 3.42 because i want somrthing that is going to be quick and perform as well thanks-dom

[b-man]

Dom,

You have to decide what type of driving you'll be doing most of the time.

A 3.23 is a great all-around highway and around-town performance gear when you're not running an overdrive trans.

Since you'll be running a fairly short rear tire (245/40-18 is 25.7" tall) the 3.42 wouldn't be as highway-friendly as the 3.23. The tire height changes your effective gear ratio, running a taller tire would drop your engine RPM on the highway some.

It's a compromise either way. Gear it too tall and it won't accelerate well, gear it too short and you'll be screaming the engine on the highway.

My brother has a '69 Firebird with a 3.42 rear gear and 235/60-15 tires (26.1" tall). He runs a mild Pontiac 350 engine that makes about 300 HP.

It's geared too short for any comfortable highway cruising at or above 65 MPH, a 3.23 would be a little better but still not the greatest.

If you'll be running around town most of the time and don't mind sacrificing some highway cruising speed, go with 3.42s.

If you'll be seeing a lot of highway trips, go with some 3.08s.

A good compromise between them would be 3.23s, which would still favor the running around on the streets.

Without an overdrive trans you can't have it all unfortunately.

Good luck, Bart

[69projectKAOS]

i think the 3.23 will be good, beacuse im probably just going to be doing alot of around town, and going to a few shows that are not that far on the highway so they sound like they would be the best. i appreciate all the help B-man thanks again -dom