So, I often get asked "Can i use the stock 6speed diff with your GR700 6-speed swap kit"... and I politely say no, you can not. But why not? Well, the 3.13 or even the 3.25 JDM gear that you can run in the 6speed supra diff (220mm) is crazy tall or crazy long depending on how you like to say it.
If you were running at 2500RPM in 6th gear with the close ratio magnum and the 3.13 6speed diff, you'd be going 100 MPH!!! That's a little higher than most people want their "cruising gear"!!! And the wide ratio magnum is even worse! That is why I sell 99.9% of the close ratio transmissions, and always recommend the close ratio.
So, what is the best option as far as a rear diff and gear? There are a few options, and it really depends on what you are planning on doing with the car.
OPTION 1 - Toyota 200MM Differential
The 200mm differential can be found in the Non-turbo MKIV Supra, Lexus GS300, and MKIV Twinturbo automatic. There are 3 ratios commonly found. The Auto TT is 3.769, the NA auto is 4.28, and the NA manual is 4.08. The 200mm is actually stronger than a lot of people think. I have a customer in Texas making over 1000WHP and he has put over 10,000 miles on his car with a stock TT auto diff (stock LSD - 200mm 3.769 gear). However, he might be a little lucky, and he only roll races. So, I do highly recommend reinforcing it by putting in a good LSD, like a OS GIKEN or Kaaz. These will get rid of the one-wheel-peel if you have an open diff and help keep the differential alive at higher HP levels. You'll also need the NA or TT Auto axles. If you are looking to drag race, I would highly recommend a slipper valve to keep the 200mm from turning to dust on a clutch drop! And don't forget to pray for those stock NA axles. More info on all Toyota differentials
OPTION 2 - Ford 8.8" Swap
The 8.8 swap kit is another option. It's a domestic rear end, parts are cheap. The LSD's are 1/3 the price of the JDM diffs, the ring and pinions are $150-200. You can basically have a fully built 8.8 pumpkin for around $800-900 bucks depending on what parts you go with. Another bonus is with the 8.8" kit you'll be forced to get stronger axles, which is not really a bad thing. And if your racing the car... well, they are gonna be needed, especially in a 6speed car. The Ford 8.8 kit seems like the best bang for the buck for the street/strip, and weekend warrior cars and those that like to have not only reliable parts, but also low cost maintenance.
OPTION 3 - Ford 9" Swap
Mr. Indestructable... the Ford 9". Set it and forget it. No more worries at the track, these beefy rears will hold up to the worst of abuse. I run one of these in my 6speed drag car and making around 2000HP, trap 195mph, and the rear just plain takes the abuse. I personally favor the Driveshaft Shop 9" conversion because they are the OG's of that swap. Again, parts are relatively cheap, and also lots of aftermarket parts are available... like Pro Gears, and spools, and anything else a drag car would want to run. I get asked if these things change the driveability, and the answer is no, if you put a wavetrac diff and a common gear in it, it will act and sound the same as a stock diff.
SO... with all that said, what is the best option?
Well... It really depends on what you are looking to do. If it's just gonna be a street car, go 200mm. Street/Strip and not making obnoxious power (less than 1000) I say go 8.8". If your making more than 1000HP and planning on running in the quarter a decent amount, just pony up and get the DSS 9" kit. It really is one of the best things I have ever done to my Supra.
So... with all that said, what is the best gear?
Well... The ratio to make the car overall gearing pretty much identical to the v160 is around 4.10. So that would be the best OEM gear to keep your car similar to a factory 6speed Supra. Personally, I like more aggressive "shorter" gearing for street cars. I run a 8.8” with 4.30 gear in my 2J-FD and it is a killer street setup with 700whp.
I might be hurting my 8.8” kit sales by saying this about the 200mm but not really, as the 8.8 probably is a better deal because building entire diff with aftermarket gear is cheaper than even a 200mm pumpkin with OS giken or Kaaz. For the drag and weekend racers the 8.8” or DSS 9” is the way to go for sure. Parts are cheap if/when anything breaks!
Attached is comparison chart I did showing overall gearing vs V160 + 3.13
So, here you can see the 4.10 is the closest match to a V160 + 3.13 (factory USDM 6speed supra overall). I have been saying this for 2 years now, but sometimes you just have to reiterate it. 4.10 4.10 4.10 !!!!!
Got questions? I am here to help. Let me get your driveline sorted... I kind of specialize in it. Not only that, but I am direct with all the big names in the business, so not only am I going to give you the best tech advice, but I can help you with the best pricing as well. Let me know what your working on, and I can help with recommendations! My contact form is here, or 866-GO-MANUAL (866-466-2682)