I picked up a JDM IS 250 last spring for my winter daily, and during the first few weeks on snow and ice I realized it was not a great fit for my location (coming from AWD BMW 535i) due to the peg leg diff. I'd heard that swapping the 3.9:1 open diff for the gear-type posi Y58 4.3:1 was an easy drop-in swap. After checking a few breakers yards in Calgary I found one for $500CAD from a low-km BRZ and snapped it up - obviously they didn't know what they had...

After swapping in, I noticed a decent improvement in acceleration (like adding 20ft-lb of torque by gearing), and the added stability and traction of limited slip is amazing. Speedometer reading on this platform is based on the ABS system, so speed reading is not changed. I'm running about 2000rpm at 100kph.

10/10 would recommend.

Edited to correct RPM at 100kph.

 

I would recommend the rear f-sport sway bars to improve handling too.

 

I leave the handling tasks to the IS F. The 250 is just for daily. But I do run the IS F OE wheels on it in the summer since I had to put rubber on them when I imported it in 2024 - get use of the Firechicken cheapos I put on for inspection.



Yeah, excuse the dirty bird.

 

How has the gas mileage changed?

 

I thought it was called a Y38 LSD? Y58 better? Does it work with a 2008 us spec is250 auto trans?

 

Probably, but the car gets amazing economy either way. Hard to tell at the moment due to cold weather, which always makes the economy drop.

 

Y38 from the first gen BRZ is the 4.10:1 LSD, Y58 from the facelifted BRZ is 4.30:1. As long as it's RWD*, it's the same case/mounting. Y38 works too but the ratio change is only about 5% vs 10% - worth it if all you want is LSD. The 4.3 gives a bit more snap.

We had the Y58 in a 2013 BRZ, then swapped it back to Y38 for sale.

*obviously AWD cars can't just go with different rear diff ratios

 

wow never knew the BRZ diffs would work on these rear ends! thanks for sharing.

 

Scion FRS, Toyota 86, and BRZ will all swap in as they all use the Toyota parts bin diffs. For IS 250 only of course.

 

So just for clarification, you just used the internals from the y58 diff then swapped it in? Or is the whole unit swappable?

 

The whole pumpkin. Bolts right in.