The Lexus IS300 is one of the coolest Japanese sport sedans ever built. Mine certainly looked the part when I first bought it, with clean body and paint, custom red seats, with interior design that aged well, a factory W55 five-speed, and a sporty, reliable 2JZ-GE under the hood. From the outside, it checked every box… until I tried doing donuts in it. That’s when reality slapped me in the face.
Mine somehow had an M85 open diff installed in the rear end. No LSD.
As they say in the Southern states, “aw hailll naw!” My IS300 is a manual car, which should mean it came with the M85 limited slip differential. Should. Instead, I had one tire spinning uselessly while the other just sat there lazily watching, with all that smooth 2JZ inline-six torque going absolutely nowhere.
For a clean manual car with so much potential, that was just embarrassing.
The three codes you need to know are the M85, M96, and M98. All three use a 7.5-inch differential carrier, which is one of the reasons parts interchangeability is so strong across this platform.
The M85 is the manual transmission car’s differential. It runs a 3.73 final drive ratio and is supposed to come with a factory limited slip differential. My car was apparently the exception to that rule, which is exactly the kind of thing that keeps used car buying interesting.
The M98 is where automatic IS300 owners hit the jackpot. This differential also carries a limited slip, but it’s a Torsen unit running a 3.91 ratio. You’ll find it in automatic cars with sport-oriented factory packages. The shorter 3.91 ratio makes the car feel more urgent off the line, and the Torsen LSD distributes torque smoothly and predictably. For an automatic IS300, the M98 is the factory configuration most people are actively trying to find.
The M96 is the one worth knowing about for a different reason. It shares the same 3.91 ratio as the M98, but it’s a completely open differential. No limited slip. It’s the most common differential in base automatic IS300s, and it’s the one that ends up welded in budget drift builds. If you’re buying a used IS300, confirming which differential is actually in the car before you hand over money is genuinely important.
Lexus IS300 Differential Gear Ratios: M85 vs M96 vs M98 Explained
The ratio difference between 3.73 and 3.91 is small on paper. Behind the wheel, you feel it.
The M85’s 3.73 gives the manual car a slightly longer-legged character. The engine doesn’t feel rushed. Highway cruising is relaxed. When you want to push, the power is there, but the car doesn’t feel like it’s constantly working. For a five-speed manual, that balance makes sense.
The 3.91 in the M98 and M96 is higher, which means the engine spins faster relative to road speed. The car responds more easily and feels snappier in city driving. For an automatic transmission, that extra urgency helps the car feel alive in a way the longer 3.73 ratio doesn’t quite deliver.
Here’s the quick reference worth saving:
| Differential | Ratio | Type | Transmission |
|---|---|---|---|
| M85 | 3.73 | LSD (or open, apparently) | Manual (W55) |
| M98 | 3.91 | Torsen LSD | Automatic |
| M96 | 3.91 | Open Diff | Automatic |
| Y38 | 4.100 | Torsen LSD | Scion FR-S / Subaru BRZ |
The photo above shows my Lexus IS300 with Y38 rear end, while cruising in 5th gear on the way to VegasDrift.
The Scion FR-S Y38 LSD is a Bolt-On Fit for the Lexus IS300
The Scion FR-S and Subaru BRZ came with the Y38 Torsen limited slip differential as standard equipment from the factory. Toyota put it in the FR-S because they understood that a well-sorted chassis with proper torque management feels fast in a way raw horsepower alone never can. The FR-S proved that to an entire generation of enthusiasts who couldn’t believe how alive a 200 horsepower car could feel through a set of corners.
The Y38 also runs a 4.100 final drive ratio, which is higher than anything that came in the IS300 factory lineup. Higher gearing in the rear diff means the engine also sits higher in its powerband during normal driving. The car pulls harder from a stop, and the throttle response feels sharper. The 2JZ-GE’s torque curve fills in lower in the rev range where you actually use it on the street. It feels like free horsepower for the IS300 without having to do any engine mods!
I couldn’t believe it when I first learned that the IS300 and Toyota Altezza platform are both dimensionally compatible with the Y38 rear end. The bolt pattern, axle flanges, carrier dimensions. It goes in as a bolt-on swap, which is the part that still surprises people when they first hear it. It just goes to show that Toyota engineers were really smart and thoughtful when it came to designing the ZN6 Toyota 86 – they probably planned it out this way! For that, we owe a big thank you to Tada san! (Tetsuya Tada was the chief engineer for the ZN6 Toyota FT86/Scion FR-S.)
One detail worth noting before you start shopping for a donor diff: manual and automatic IS300 differentials can require different inner axle stubs. Confirm your axle hardware matches what you’re installing before you buy. It’s the kind of thing that turns a straightforward afternoon job into an unnecessary parts chase if you miss it.
Torsen vs. Clutch-Type LSD: What’s the Diff?
The Y38 and the factory IS300 LSDs are all helical gear Torsen differentials, but the FR-S unit carries a more aggressive torque bias ratio than the stock M85. The improvement was immediately noticeable the first time I drove the car after the swap. The rear plants under power instead of searching for traction. Corner exits are cleaner. The chassis finally feels like it’s working with the engine instead of against it.
For context on where the Torsen sits in the bigger picture, here’s how the main differential types compare:
Torsen LSD (Helical Gear) Works entirely through mechanical worm gear principles. No clutch packs, no break-in period, no maintenance beyond regular fluid changes. Torque transfers smoothly and progressively toward the gripping wheel. The engagement is transparent and confidence-inspiring. It works without asking for anything in return.
Clutch-Type LSD (Kaaz, OS Giken, Cusco, ATS) Uses friction plates that lock mechanically when torque split exceeds a set threshold. The locking behavior is tunable through preload adjustment and plate count. At the limit, a properly set clutch-type LSD delivers more aggressive and controllable locking than any Torsen unit. OS Giken’s Super Lock is one of the most respected options in the IS300 and Altezza community, and the price reflects that. Same thing with Cusco. The Kaaz LSD offers excellent 1.5-way and 2-way configurations for drivers who want precise control over locking behavior both entering and exiting corners.
Eventually, I’d like to upgrade to a clutch type LSD in my IS300, because I noticed a huge difference when I upgraded from the stock Toyota LSD in my AE86 GT-S and got my first Kaaz LSD installed. I went with a 2-way in my kouki AE86, then got a 1.5 way Kaaz installed in my zenki AE86. I’ll most likely try to get a a Kaaz LSD when I have the budget to upgrade the IS300, since I have a history of working with them. (If you’re interested in a Kaaz LSD, email me and I’ll try to help out! ( motormavens AT gmail )
Clutch-type LSDs do require a strict break-in procedure when you buy them brand new. You need to load the diff, and drive in a figure 8 pattern for quite some time, then change the gear oil in the LSD. Improper break-in can make a clutch type LSD produce driveline chatter during tight low-speed turns.
Another drawback to clutch type LSDs, is they cost significantly more than an OEM torsen diff, as can be expected. OS Giken units start around $1,450-1600. Kaaz units sit at around $1500, but sometimes I can get them for a little bit cheaper. For time attack, circuit racing, or a dedicated drift build, that investment makes complete sense. For a street-driven IS300 that sees occasional track days, the factory Y38 torsen LSD swap delivers “good enough” results at a fraction of the price.
Open Differential No torque transfer. One wheel spins, forward motion suffers. Fine for the grocery store run, genuinely limiting everywhere else. Ask me how I know. It’s embarrassing. You don’t want this in your life.
Hooked on LSD at Garage Boso
To be honest, it took me a long time to get the Y38 LSD installed into my car after acquiring it. After buying it from Jason, I was super busy with other things (I was working for McLaren at the time and still tied up moving my cars and parts from Los Angeles to Vegas), so I didn’t have time to mess with any of my cars. I haven’t crawled under one of my cars to install an LSD in 15 years, so I asked my friend Ross Petty from Garage Boso for help.
Ross is a former Formula Drift pro driver, which means he has spent more time thinking about how rear ends behave under load than most people spend thinking about cars in general. When someone with that background agrees to work on your build, you clear your schedule and show up on time.
Ross confirmed the fitment, dropped in the Y38, filled the gear oil, installed new Swift Springs onto my Apex’i coilovers, and had everything sorted for me. He took it for a test drive after installing it, saying “It feels like it should have come from the factory this way. This thing rips now!”
That’s exactly what I wanted to hear!
IS300 Y38 LSD Swap: Real World Driving Impressions
The first drive after Ross finished the install told the whole story. The rear plants. The throttle drives the car forward. The 2JZ-GE’s torque, which always felt slightly wasted through the open M85 under hard use, now reaches the road.
The 4.100 final drive also makes the car feel noticeably more responsive in everyday driving. The engine sits higher in its powerband, so it pulls harder from a stoplight. At highway speeds, RPM runs slightly higher than stock, which is worth knowing if long freeway commutes are a regular part of your life. For the kind of driving I do, it’s fine and I’m not worried about it. I used to daily drive an AE86 every day for crying out loud. The IS300 feels ultra luxurious in comparison.
The car that checked every visual box before now checks the driving ones too.
The naturally aspirated IS300 rewards the people who take the time to understand it. The 2JZ-GE gets all the attention, but the differential is where an NA car’s real character lives. It’s what decides how that power actually reaches the road, and how much confidence the car gives you when you’re asking it for everything it has.
Whether you’re chasing an M98 to replace an automatic open diff, hunting a proper M85 LSD for a manual swap, or going the Y38 route like I did, getting the rear end right is one of the best investments you can make in this platform. A proper LSD transforms the feeling of the IS300 without changing a single thing about the character that made you want one in the first place.
Thanks to Rommel Lalunio for the info, Jason Wilcox for the limited slip, and to Ross Petty for doing the install. The car is much better for it. Now that it’s a lot more capable, it’s time for me to spend more time at the race track and getting more seat time at VegasDrift!
I’m definitely not done with this thing. Stay tuned to MotorMavens for the next chapter of the Project IS300 build.
:: Antonio Alvendia
Instagram: @AntonioSureshot • @MOTORMAVENS
www.antoniosureshot.com
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