How-To: Change final drive ratio (relatively) cheaply
 

I just recently completed a final gear swap in my FR-S a couple days ago and have another thread going on that. What I want to do here is give the recipe I followed all in one place.

For reference, when I refer to a "carrier", I mean specifically the whole pumpkin assembly with the actual differential installed in it. Carrier, pumpkin, third member, same thing.

Note for turbo guys: If you're looking to swap to a taller gear ratio, just go get yourself a carrier from an IS300. You want either an M98 (3.9 final drive) or M85 (3.7) code. These are the LSD variants for the automatic and manual IS300, respectively. Both of these are a direct swap into our cars, the only trick is to pop out the axle stubs (the wheels on the sides) as our axles simply spline straight into the carrier. You still may consider overhauling the carrier/differential depending on the condition yours is in, but otherwise you can stop reading here.

OPTIONAL - Donor Carrier
What I did to minimize my downtime was to obtain one of the carriers mentioned in the note above, and had my gears installed in that. Added bonus of having a second working carrier in case something goes Horribly Wrong.

Optional Cost: ~$300 depending on condition

Meat and Potatoes - Ring and Pinion
Give Spence a call at weirperformance.com. Tell him that you want an 8" ring and pinion set for a mark 3 Supra and that you intend to use it in an IS300 or FR-S/BRZ carrier. Then just make up your mind what ratio you want. He also sells a shim kit that I highly recommend for $15. It has all the shims you could possibly need to get the install right.

Cost: $225 for ring and pinion, $15 for shims, plus shipping

Fiddly Bits - Seals and Nuts
For these I just visited a couple different Toyota parts distributor sites. Here's the breakdown of what I got for my installation:

• 90179-20018 Pinion nut
• 41231-24010 Pinion spacer
• 90311-43009 Side seals (x 2)
• 41222-22010 Ring gear lock tabs (x 5)
• 90311-38070 Pinion seal

Note: My carrier was basically brand new so I didn't need to replace any bearings. These tend to be rather expensive (pinion bearing I think was about ~$80, not sure on side bearings). If you think you need to do the bearings, you may as well buy Spence's installation kit. It should contain all of these parts plus the bearings. Not sure on the price there though, but it's on par with acquiring the parts individually.

Other note: If you're looking to make Balls-Out Torques, you may consider getting a solid spacer in lieu of the OEM pinion spacer. Also available from Weir, also part of the installation kit.

Cost: ~$100 without bearings, plus shipping

Putting it together - Labor Cost
4 hours for the ring and pinion install, 2-3 hours for carrier install. I'd recommend a dedicated differential shop for this just because they should be guaranteed to get the setup dead on. Labor costs vary widely, and you could easily save a couple hundred bucks by installing the carrier in your car yourself. I chose to let the Professionals handle it all.

Cost: ~$700 if the shop does everything, ~$400 if you install your own carrier, $0 if you're a Professional

Price Breakdowns

• Bare Minimum (just required parts, all installations done yourself): $350
• Bare Minimum + ring and pinion installed by shop: ~$750
• My Total (spare carrier, no bearings, all installation handled by shop): ~$1100
• Theoretical Max (all parts, all services done by shop): ~$1500

So there you have it! Let me know if you think I missed anything or need further clarification on something. Big thanks to @37, @Jeff Lange and anyone else I may have forgotten for helping me figure out what ratio I wanted and what installation parts to buy.

Excellent post, thank you!

I'm not sure what the pinion spacer is could you clarify that for me? .

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Picture the pinion as the gear on the end of the driveshaft that spins the ring gear. You don't want it pushing off the ring gear and sliding back up the driveshaft. A solid spacer to hold it in place makes that impossible. When I say driveshaft, I obviously mean the assembly in the carrier that the driveshaft connects to, but you get the idea.

I will be doing this eventually. Thanks for the post!

Amazing post! One question though. I live in the Tacoma/Seattle area too and was wondering where you found your carrier at? Local, or did you order online? Thanks!

If you're looking to buy local, do a search on car-part.com. There's always eBay if there's nothing around, but these suckers are HEAVY so shipping will probably be awful.

Ok, cool, I understand that. What's the OEM spacer made of if it isn't solid?

The OEM spacer is still made of steel, but it's meant to be "crushed down" when putting everything back together. Hence the alternative term, crush sleeve. A solid spacer is much more substantial and doesn't deform during reassembly.

Golden thread!

Part numbers are a great help, thanks again

Total noob question, but when searching on car-part does it matter if it is locking or non-locking? Are you able to slap the LSD from your diff into the carrier? If this is stupid sorry :iono:

The carrier itself is identical between the locking and non-locking versions, so theoretically yes, you could take your FR-S/BRZ differential out and have it swapped into an IS300 carrier. However that sort of defeats the purpose of getting the second carrier because you still have to take yours apart; you may as well just install the ring and pinion directly into your carrier. Also I didn't do that, so I'm not 100% on whether that will actually work.

Open differentials suck. You definitely want a LSD version if at all possible.

Great work, this will be very helpful to many (including myself haha)!

I will definitely be doing this as well. Thanks OP for all the info!!

Thanks for the post. Ill be doing this aftet my wheels