Considering going to 3.727 gear/pinion in 6MT

[ESBjiujitsu]

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

[serialk11r]

Originally Posted by SpeedR

ya the auto could use lower gearing for better acceleration. I would do this in a second.

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.

[Calum]

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.

[HotLavaMan]

Originally Posted by Calum

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.

[serialk11r]

Originally Posted by HotLavaMan

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.

[bimmerboy]

+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

[earthsscum]

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.

[smbrm]

Originally Posted by Jeff Lange

Consider this:

http://www.lexuspartsnow.com/parts/lexus-final-gear-kit-differential-rear~41201-80050.html

Jeff

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

[Jeff Lange]

Ring & pinion set only.

Jeff

[smbrm]

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

[azian_advanced]

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.




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.

[Dave-ROR]

Originally Posted by Jeff Lange

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

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

[HotLavaMan]

Originally Posted by smbrm

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

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.

[HotLavaMan]

Originally Posted by azian_advanced

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.

<chart>


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.

Thanks! that is a really useful chart.

[Dave-ROR]

Originally Posted by HotLavaMan

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.

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.

[smbrm]

Originally Posted by azian_advanced

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.

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.

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.

[Jeff Lange]

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

Jeff

[smbrm]

Originally Posted by Jeff Lange

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?

[Jeff Lange]

Probably, but generally it is not something that can be changed/calibrated.

Jeff

[azian_advanced]

Originally Posted by Jeff Lange

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

Jeff

wow.. the speed sensor for the speedometer was always on the transmission for most cars. never knew speed sensors are now based from the abs sensors.

what happens when the car is turning and one wheel is turning faster/slower than the other? i'm curious to know how the speedometer compensates for this.

[Mitch]

Originally Posted by serialk11r

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.

Base mpg: 30.0 mile / gal
New diff: 5% improvement
New mpg: 31.5 mile / gal

Premium fuel cost: $4.00 / gal

Base gas cost: $0.133 / mile
New gas cost: $0.127 / mile
Savings: $0.00635 / mile

Payback mileage (miles to break even, every mile after is actual savings): 157.5 miles per $1 spent modifying the gear ratio.
In other words, if you spend the $1106 mentioned earlier, it would take 174,195 miles before you broke even on the cost of the differential.

And for a little sensitivity study...
Payback mileage as premium fuel cost changes:

$3.00 / gal : 210.0 miles per $1 spent modifying the gear ratio
$3.50 / gal : 180.0 miles per $1 spent modifying the gear ratio
$4.00 / gal : 157.5 miles per $1 spent modifying the gear ratio
$4.50 / gal : 140.0 miles per $1 spent modifying the gear ratio
$5.00 / gal : 126.6 miles per $1 spent modifying the gear ratio

[serialk11r]

Originally Posted by Mitch

Base mpg: 30.0 mile / gal
New diff: 5% improvement
New mpg: 31.5 mile / gal

Premium fuel cost: $4.00 / gal

Base gas cost: $0.133 / mile
New gas cost: $0.127 / mile
Savings: $0.00635 / mile

Payback mileage (miles to break even, every mile after is actual savings): 157.5 miles per $1 spent modifying the gear ratio.
In other words, if you spend the $1106 mentioned earlier, it would take 174,195 miles before you broke even on the cost of the differential.

Yes, which is why the cost needs to be taken into account obviously. If it weren't so expensive to get custom gearsets made then a 6th gear swap that drops revs say 15-20% rather than just 10 would make a lot more sense.