Hey all,
Currently building a drift car and need some help with gearing.

I plan on running a 370zhr transmission JK40C with a Ford 8.8IRS. The Diff i bought had 3.73 LSD gears. I’m still debating between a 13b Semi PP/ Street Port or 2JZ for engine. Ive never messed with gearing before I’ve always stuck with factory gearbox and factory rear gear. What would be a proper rear gear for the JK40C.

Wheel Size: 255/35/18 18x9.5

 

You will want to go 4.3 or higher. FCs where i am have the 370z box with 4.3 T2 rear end and its just fine. 3.73 is too high i think.

 

there’s a thread in here on that trans conversion and the gear ratios relative to factory are discussed in it

edit: here it is, relevant part might be about 3/4 of the way towards the end

https://www.rx7club.com/rotary-car-p...z-13b-1090772/

4.3 is going to be pretty low, in the lower gears especially, but it may be what you want. Just depends I suppose. Because the ratio range on this 6 spd is quite a bit wider than the Mazda trans. As I posted in that thread, a 3.73 ratio gets you close to the OE trans with 4.300 ratio, just closer ratio and a better cruising gear which you likely don't need. So I suspect you might want lower gearing ... maybe 3.909 or 4.100 will do, but 4.300 or higher it you need a granny gear to get going.

If you need some assistance running the numbers through a gearing calculator you can feel free to pm me.
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compare the final drive in your ford diff to the z diff along with the tire diameter you plan on running.
there are a few transmission gearing calculators online that you can use to figure out what speeds and rpms will be. google for those.