Im going to be rebuilding my borg-warner 9-bolt. it currently has 3.45 gearing and I want to change that, but I don't know too much about how much difference each size would make.
I can get 3.7 gears for 199.99
or I can go up to 3.9 for 275.99


this is for my daily summer driver. still like it to be quick though
any help

 

Choosing the "best" gears for you involves a lot of factors. Going to 3.70s won't feel dramatically better, and the 3.90s will be annoying on the open interstate. But if you've built an engine that doesn't peak until 6500, for example, then you'd be better off with the 3.90s.
I'd leave the 3.45s in there unless you're abandoning the TPI. The 3.90s will make first gear nearly useless.

 

Yea, you probably have the best gears in there now for the TPI and 700R4.

 

 

3.4X gear is the overall best gear for most of the street use mild performance use cars.

Gives a good balance of RPM and accel.

Like mentioned unless you are using much better/bigger heads, cam, etc that rises the peak power points into the 5500-6500 rpm and above ranges, there is really no reason or need in 3.7X or 3.9X gears

 

Wish I had the 3.45 or3.42 instead. I just got a good deal on my rearend with the 3.73s in it. 3.42s are perfect. Peppy but not too steep on the hwy