Guesses about Gear Ratios and Wheel Torque

[Maxim]

I dunno. The gearing doesn't seem all that tall to me. Remember what kind of torque curve we're dealing with.

It's not the peaky Renesis engine or a Honda after all. It's a Subaru with some fancy Toyota injection stuff hooked up.

[Ryephile]

So, as it turns out, 70 MPH in 6th [with 6MT USDM] is about 3100 RPM, and 80 MPH in 6th is about 3500 RPM. The huge difference in the 6th gears between manual and auto may actually account for the big EPA estimated fuel economy estimates. We'll see if the relatively high revs of the manual indeed kills the fuel economy in the real world, but I can envision people swapping in the 3.727:1 rear gears from the 16" wheel markets that use the car strictly as a highway-biased DD. It's good for about a 10% RPM drop, which would make for a noticeable fuel economy gain [and accel loss].

[Dimman]

Originally Posted by Ryephile

So, as it turns out, 70 MPH in 6th [with 6MT USDM] is about 3100 RPM, and 80 MPH in 6th is about 3500 RPM. The huge difference in the 6th gears between manual and auto may actually account for the big EPA estimated fuel economy estimates. We'll see if the relatively high revs of the manual indeed kills the fuel economy in the real world, but I can envision people swapping in the 3.727:1 rear gears from the 16" wheel markets that use the car strictly as a highway-biased DD. It's good for about a 10% RPM drop, which would make for a noticeable fuel economy gain [and accel loss].

If that diff is strong it could also be better starting point for guys that go FI, too.

[old greg]

Originally Posted by Ryephile

I can envision people swapping in the 3.727:1 rear gears from the 16" wheel markets that use the car strictly as a highway-biased DD. It's good for about a 10% RPM drop, which would make for a noticeable fuel economy gain [and accel loss].

Seems like it would be easier to get from a junkyard stateside. Since it is from all appearances an IS pumpkin, it should be possible to use the 3.73:1 ring and pinion from a IS250 or IS300 (both in manual), or from a MkIII Supra Turbo.

[Dimman]

Originally Posted by old greg

Seems like it would be easier to get from a junkyard stateside. Since it is from all appearances an IS pumpkin, it should be possible to use the 3.73:1 ring and pinion from a IS250 or IS300 (both in manual), or from a MkIII Supra Turbo.

Wait what? Really? This goes all the way back to Mk3 Supra-based components? Forced induction guys are going to be stealing Mk3 Turbo (all clutch-pack LSDs) diffs. Too bad the mileage is enormous on most of them...

(My buddy finally broke his with ~800ish dynojet whp after a few dozen drag launches on slicks...)

[old greg]

Originally Posted by Dimman

Wait what? Really? This goes all the way back to Mk3 Supra-based components? Forced induction guys are going to be stealing Mk3 Turbo (all clutch-pack LSDs) diffs. Too bad the mileage is enormous on most of them...

(My buddy finally broke his with ~800ish dynojet whp after a few dozen drag launches on slicks...)

The pumpkin is different, but the guts are the same. Or at least that is my understanding.

[Deslock]

Since this old thread was revitalized, here's some more data:

gear   MT4.1    MT3.7    AT
1      3.626    3.626    3.538
2      2.188    2.188    2.060
3      1.541    1.541    1.404
4      1.213    1.213    1.000
5      1.000    1.000    0.713
6      0.767    0.767    0.582
rev    3.437    3.437    3.168
DR     4.100    3.727    4.100

USDM gets MT4.1 and AT. Tires are 215/45R17.

Approximate MPH at redline (7400 RPM):

gear   MT4.1  MT3.7  AT
1      36     40     37
2      60     66     64
3      85     93     93
4     108    119    131
5     131    144*   184*
6     171*   188*   225*

* = higher than rumored top speed

Note: Assumes "rolling radius" is 1% smaller than the 
calculated radius, but in reality rolling radius varies with speed.

Approximate peak wheel lbf*ft (6000 RPM):

gear   MT4.1  MT3.7  AT
1      1910   1740   1860
2      1150   1050   1090
3       810    740    740
4       640    580    530
5       530    480    380
6       400    370    310

Note: Rounded to nearest 10 lbf*ft.

[serialk11r]

Originally Posted by Dimman

Wait what? Really? This goes all the way back to Mk3 Supra-based components? Forced induction guys are going to be stealing Mk3 Turbo (all clutch-pack LSDs) diffs. Too bad the mileage is enormous on most of them...

(My buddy finally broke his with ~800ish dynojet whp after a few dozen drag launches on slicks...)

Hmmm now that I finally noticed, it looks like the diff is same as in older cars, and the transmissions are basically off the shelf too (well, same ratios as some older cars). Maybe they wanted to get the car out sooner so they just used off the shelf parts whenever they could.

[ahausheer]

Originally Posted by Allch Chcar

It's a street car, Cereal. Top gear is always taller than necessary for MPG, NVH, etc. 5th gear is the top speed gear. If you want to use 6th gear for top speed you need to use a much shorter axle gearing. >

Can you explain this? When I'm on my bicycle I can go fastest in top gear. What is the difference in a car?

[serialk11r]

Originally Posted by ahausheer

Can you explain this? When I'm on my bicycle I can go fastest in top gear. What is the difference in a car?

He misunderstood my post originally, but imagine your bike had a gear so tall that you couldn't pedal anymore. If you could identify the rate at which you could put the most power to the pedals, and then geared the bike to match the road load to your legs' "maximum output", then that would be the gear in which you go at top speed. If it's any longer, you can't produce enough work to overcome the drag forces and whatever.

But on a car you have another problem, efficiency is very bad at low loads. An engine typically has a huge excess of power available at cruising speed, so you use taller gears to reduce the engine speed and increase the load to run efficiently.

[Ryephile]

Deslock: Thanks for posting the numbers. I literally just finished fiddling with my gearing and torque spreadsheet when I clicked this thread. :happy0180:

[ahausheer]

Originally Posted by serialk11r

He misunderstood my post originally, but imagine your bike had a gear so tall that you couldn't pedal anymore. If you could identify the rate at which you could put the most power to the pedals, and then geared the bike to match the road load to your legs' "maximum output", then that would be the gear in which you go at top speed. If it's any longer, you can't produce enough work to overcome the drag forces and whatever.

But on a car you have another problem, efficiency is very bad at low loads. An engine typically has a huge excess of power available at cruising speed, so you use taller gears to reduce the engine speed and increase the load to run efficiently.

Ah I get it, when I peddle up a steep hill on my bike my fastest speed will not be top gear because I don't have the power to move that gear, so I am forced to select a lower one, ok makes a bit more sense now. sorry to get off topic, I just like to learn about cars.

[Deslock]

Originally Posted by Ryephile

Deslock: Thanks for posting the numbers. I literally just finished fiddling with my gearing and torque spreadsheet when I clicked this thread. :happy0180:

Sure thing. BTW, I went back and rounded the MPH because I'm not sure how to calculate actual "rolling radius" (how much tires squish varies slightly with speed). So I used 1%, and thus these numbers are approximate.

[serialk11r]

Hey guys sorry but I just noticed, the reported top speed of the car is said to be 143mph...
Yet 5th maxes out at 131! So 6th gear is the top speed gear. wuuuuutt is going on