Info about the s15 6speed trany

[chibinissan]

Wats the best rear diff ratio for the s15 6speed trany
I got a 4.100 but I think its too much I dont want to cut it to put it in and its not good

[homeslicej2]

?? Wait, what exactly are you asking, bc your last sentence doesn't make sense? If you want to know what the best rear-end gear ratio would be to run with the trans that's better determined by your motor, how it's setup (assuming you have the option to get a custom ratio), what your goals/usage for the car will be, and for racing, each track. Look at the S15's gear ratios, your rear-end ratio, and your engine's powerband and RPM range. You can also take into account tire diameter if you're running a much smaller or larger size (say for drag racing). There's a start.

[chibinissan]

track and street use im doin a sleeper (looks like stock outside)
i got an d21 truck and im swaping the engine for s15 with a 6speed trans
i want to know wats the best rear-end gear ratio for it im goin to put stock size tires 14/75
i got a 4.10 diff al ready but i want to make sure before i cut it to fit the truck ( its too wide )

[Seishuku]

Why not use the stock axle, it's a H233 right? Probably a 4.3:1, which would be awesome for laying down torque, though top speed would suffer (top out around 135MPH at 7200RPM I think).

[chibinissan]

thats too much for street
do you know the stock rear end ration on the s15 6speed silvia?

[Seishuku]

I think the stock diff on the S15 is a 4.36, I would have to look it up though.

I daily drive my S12 with a 4.11:1 rear end, and it's not too bad... On the highway it was turning fairly high on the revs (about 3600), but that was with the stock trans (had a .853:1 5th gear) and now I have an S13 trans so it's not too bad anymore (.759:1 5th).
But I love the acceleration from having a short final gear, even though top speed suffers... I've been actually thinking of going to a 4.36 rear end.

And actually, with your truck being a heavier vehicle, a shorter final gear would be better anyway... Makes it easier on the engine to move all that weight.