Correct, you will lose some time 0-60. That's why people don't typically go over the 3.73 gears which shifts into 3rd at around 58.5mph. However, many add a high stall torque converter and go with slightly larger wheels where the overall diameter is a bit larger than stock. This will then make you shift around 60mph again but now you have the high stall TC to get those big wheels moving quicker off the line plus the 3.73 comes as a limited slip (supra rear end) for added traction. The 3.92 gears from a GS300 is only an open diff providing less traction. I did a lot of research on this and have the perfect combo of all these items regarding rear diff, torque converter and slightly larger wheels. It goes good.
Originally Posted by RoMiLex
Everybody keeps talking about the 0-60 improvement. Technically i think you end up a tad slower because the gear change causes you to need third before 60 where stock it shifts after 60. But anytime else it makes you quicker. You loose top speed though. Ive read that 1/4 mile times improve about .300-.400.
As far as the IS300 diff I read that the bolt circle is 150mm and the GS400 circle is 160mm. I think. But the gear itself is 200mm in both cars. So as long as the splines coming out of the carrier are the same on both cars it should work. IS300 guts in a GS400 differential housing. Bam, 3.92 w/ limited slip.