I blew my diff in my 07 and am going to have to send it in to be rebuilt... It is my DD and I do drive it on the freeway so It would be nice to get a little bit better gas mileage. Im Supercharged as well... would 3.91 be any benefit to me. Would it be faster in a certain MPH range as ill be in boost longer????


Or will it just make me slower all around...

 

It will make you slower. Smaller gears are always faster most of the time.

2 exceptions

1. Drag cars go with smaller gears or a lower final drive because it makes them finish the 1/4 mile in 4th gear so they don't have to shift to 5th gear

2. If you turbo and you have about over 500whp. You will have tractions problems in 1st and 2nd gear. So with bigger gears/smaller final drive the car can have better traction and can be faster.


In my opinion smaller gear/lower final drive will only be benifical if your over 500whp. With your supercharger and on pump gas I doubt you have traction problems.

 

What were you doing when the diff blew?


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Taking a nice 90* corner at about 50mph. Mid corner it pops and limps to the side of the road previous owner must of beat on it or something.

 

I'm running the 3.90s and they are great. They drop the rpms on the freeway nicely.

I took my supercharger off for a few weeks and it ran out fine NA.

What kind of power are you making?

 

right now 300 but will be closer to 380 after I get the diff finished up.

Did you do any before and after runs to compare? I feel like they would be nice for cruzing but if its going to make me go backward in my attempt to make the car quicker its probably not worth the extra $500


Edit: not exactly trying to get rid of torque either...

 

Which gear set is 3.9?

 

Mazda is 3.9.

 

If you're boosted the amount it'd make you slower is really negligible imo, especially on a road course/track. The AP2 gearing I find very short (it's shorter than ap1), yet it also has a lower redline. I would go 3.9 without thinking about it. Heck, if I boosted my car, I'd consider going to 3.7. Plug the ratios in an online calculator to calculate your max speed in each gear for a certain rpm and decide for yourself.

 

I just plugged it in a gear calculator and it really doesn't make a huge amount of difference.