Considering going to 3.727 gear/pinion in 6MT
 

Has anyone heard definitively that the rear diff is the same one as a Lexus IS250? I'm very interested in going from the 4.10 to the 3.727 gearing but don't want to waste money on a special order part that I probably would have a hard time returning if it isn't the correct one.

I'm trying to get lower RPM in 6th for highway cruising. Sure, I could just replace the gearing on 6th but that seems very expensive route to go. I don't think I'll mind making the other gears a bit taller.

Anyone know for sure one way or another? Other option is to try to order the JDM part from Japan as the GT86 has the 3.727 is an option.

Also : anyone know how if it's easy to adjust speedo for different ratio? Hopefully just a firmware setting?

Thanks

sorry for thread jacking but may i ask why? is the gas saving that significant or is there other reasons that I miss? I'm surprised at the high gear in 6th myself but slowly getting used to being able to quickly pass car in cruise control without downshifting, i think they made this conscious decision to make it feels like a sport car.

All of the reviews say this car is a hair under-powered as it is. Do you have your car yet? Have you driven it much on the highway? I thought my Civic Si revved absurdly high on the highway, but I got used to it quickly and the fuel economy was acceptable. That's another consideration: what's the payback period for the cost of swapping a differential vs the actual fuel savings you'd see?

It's a fair question and one that I reckon many who go forced induction will be asking.

I've got about 650 miles on mine. I find the current pickup adequate and don't mind down shifting to pass. My goal is simply to cruise at a slightly lower RPM (increase in gas mileage and decrease in noise). It's not bad as it is now and as you say, it's probably a long payback for gas savings to pay for it. I find the gearing a bit short from what I'm used to so that's why I'm looking into it.

subscribed. im interested in this too

I can confirm the differential is the exact same as the IS300 and IS250.

:)

Jeff

Awesome. Thanks much Jeff.

3.727 would have been a much better compromise for the manual for street use. 4.1 appears to have been selected more for the track than the street.

With the 4.1, on the manual I believe you will be doing a lot of shifting in city driving in the 1-4 gears, while on the AT I believe you will be doing more shifting between 4-6 on the highway.

How big of an investment is an IS250 diff?

If you want to get a complete differential, you'll want one from the IS300 with optional LSD. IS250's do not have LSD.

IS300/250's come with 3.73 (MT) or 3.91 (AT) gearing, but only the IS300 has the optional Torsen T2 LSD that the FR-S/BRZ has.

Jeff

I reckon a lot of IS guys will switch to the 4.1 ratio. :)

I'm interested in this but was hoping to go the opposite on the AT. I'd like to go to a 4.4 or somewhere in the ballpark. If anyone has any info please share. Thank you!

I spoke with my local Lexus dealer parts department. It's either a 41039-22010 or 41039-35010 and he's not sure which one would be the correct one. Price is either $172 or $197 and will take about a week to get here from central Lexus parts depot. So more research required.

Jeff Lange, do you have any suggestions on finding out which Lexus part number it would be?

5th & 6th could be lower on th AT

According to DadHawk, his AT is running at 2350 at 70 MPH whereas my MT is running a bit over 3000 rpm at same speed. That is quite a big difference.

And that is why I'm looking to get the 3.727... it would split the difference between the two.

No, those are not what you are looking for, that is the differential internal gear set for a non-LSD open differential. Both of those are the wrong part number. Not sure why they would have looked those up for you.

41201-80050 - Ring & Pinion Gear Set, 3.727:1
41201-80052 - Ring & Pinion Gear Set, 3.909:1
41201-19605 - Ring & Pinion Gear Set, 4.100:1
41201-29606 - Ring & Pinion Gear Set, 4.300:1

Pricing is probably going to be quite a bit higher than what you were quoted for those wrong parts.

Jeff

Thanks much Jeff. Great info. Do you know if those part kits would also include the appropriate "crush sleeve" (according to my mechanic this would also need to be replaced) or is that yet another part to be ordered?

The kit includes the ring gear, pinion gear, pinion seal, pinion stake nut, crush sleeve, ring gear bolts and ring gear bolt locking plates.

The 4.300:1 gear kit listed above is for a different model, so the pinion seal is different for that kit. If you went 4.3, you'll probably also need a new pinion seal.

Jeff

Thanks for all the great info Jeff. Then looks like the 41201-80050 is the one I'm after and has all required parts.

That's correct. :)

Jeff

Local pricing through my Lexus dealer for 41201-80050 is $1106... ouch.

That doesn't include tax and actually having it installed. Hm. I may reconsider priority of changing that out at that price.

Consider this:

http://www.lexuspartsnow.com/parts/l...201-80050.html

Jeff

this thread is full of good info!!!!! :thanks:

Only problem is, you still have the crappy ginormous gap between the ratios, so you will only get marginally better 1st gear acceleration :/

Apparently on some Japanese site the 3.727 gets about 5% better fuel economy on their test cycle, which is probably around what you can expect cruising on the highway. If you drive a lot the savings could add up quickly.

How much gas are people expecting to save by doing this? Why not just disconnect the sound tube after the diaphrame? That would cut down on the noise at least.

Well in theory you could on the MT get the same mileage as the AT which would move it from 30 to 34 or about 13% (EPA estimates of course). I don't think it's going to be that high as the total gear ratio will still be shorter than the AT even with the 3.727. But 10% might be within reach.

Naw, 3.727 is 10% lower than 4.1, you won't see 10% fuel economy increase from that. The MT revving 30% higher than the AT in 6th is likely worth 10-15% fuel economy depending on speed. 5% would be pretty good already.

+1 this is a great idea, Cruising around 70mph would bring the rpms down a bit as well as lengthen the ratios a little. Should come in handy once power upgrades are available :)

very intresting thread! Thanks for the info, this car has the potential to be more fuel efficient with a few tweaks especially considering how light it is.

I wonder if that is for the guts or the whole casing as well?

Ring & pinion set only.

Jeff

So a rebuild of the guts required then. How would one recalibrate the speedometer, or do you even have to if speed is taken from wheel sensors?

I figure the following:

4.1 approximate Ratio relationship MT/AT: 1~=1,2<2,3<3,4>3,5=4,6<5,6<6
3.727 approximate Ratio relationship MT/AT: 1>1,2>2,3=3,4<4,5<5,6>5,6<6

Here's the difference in speed while cruising at 75 mph between the 3.727 (shown as BRZ P-MT FD1) and 4.1 final drive gears (shown as BRZ P-MT FD2) between the manual transmissions. The canadian version is here.

http://i10.photobucket.com/albums/a1...mbined-MPH.png


depending how much time you spend cruising on the freeway, the 300 rpm difference between the FD gears may not be worth it for the fuel economy difference and the payback period of buying and installing the FD.

but if the transmission gear shafts between the AT & MT are swappable, then you're looking at a much better difference.

A 4.3... Now I'm interested a lot more in this thread!!

Compared to my other cars this thing is quiet and has low revs on the highway :)

It is not taken from wheel sensors. I spoke to a guy who had replaced his wheels and did not have the flat sensors in the wheels and the car worked fine so these are not required for speedometer. I'm hoping it's just a firmware setting but don't know.

Thanks! that is a really useful chart.

A wheel speed sensor isn't in the wheels, that's the TPMS sensor. This car could read off on of the ABS rings, or off the trans output (which would cause a bad reading with a replaced FD), or the diff I guess..

The FSM should have that info but I don't have the FSM yet. I should buy techinfo access to get it though.

Good stuff. I just finished my own chart so you beat me to it. Do we assume that L-AT is for the Lexus?

"....If you like to row gears, this car is for you!....." from http://www.ft86club.com/forums/showthread.php?t=7459 in addition to other comments
seems to sum it up for me.

Having driven the AT, 3rd gear is a very flexible sweet spot in terms of city driveability from 40-70 km. In MT it should be 4th! These will keep you on the lower torque peak.

The MT will be a city shifter(more shifting in city driving). Not sure the 3.727 will change that.
The AT will be a highway cruising shifter(for passing and steep grades)!

In restrospect they probably should have put the 3.727 in the MT and the 4.3 in the AT. They are in the parts bin after all.

Speed signal is supplied by the ABS sensors. Changing gearing will not affect the speedometer accuracy.

Jeff

Thanks, I would have thought that would be the easier way to do it. But it would still be affected by tire size. There is probably a tire size constant in the programming somewhere?