Guesses about Gear Ratios and Wheel Torque
 

This Subaru video shows 97 km/hr @ 7300 and 61 km/hr @ 3200:
http://youtu.be/eP3SR1m_aRY

That means redline is approximately at:

2nd 61 MPH
3rd 88 MPH

If we assume typical differences between gear ratios, we can speculate:

1st 40 MPH
4th 116 MPH
5th 144 MPH
6th 181 MPH

For highway cruising, that puts 75 MPH in 6th at 3100 RPM.

Peak wheel torque per gear, rounded to the nearest 10 lbf*ft, assuming 15% drivetrain loss:

1st 1700 lbf*ft
2nd 1110 lbf*ft
3rd 770 lbf*ft
4th 590 lbf*ft
5th 470 lbf*ft
6th 380 lbf*ft

Note that 4th-6th seem a little tall, but that's because 3rd is a bit tall in the video. Before noticing that 3rd was in the video, my guesses were:

3rd 86 MPH
4th 113 MPH
5th 141 MPH
6th 176 MPH

Obviously redline won't be reached in 6th gear, and possibly not in 5th if rumors of a 136 MPH top speed are true. However, some of those rumors claim top speed is due to fuel cutoff (supposedly at 7450 RPM), which raises some questions.

Nah, it'll hit 141 just fine.

Maybe. I read a while ago that top speed would be a little less. I don't expect to take it that high anyway.

Edit: Note that I revised 5th to redline at 144 rather than 141

I wonder why the wheel torque per gear never shows up on a dyno?

Because they only dyno in one gear, usually whichever is closest to 1:1 (4th or 5th). So the dyno is getting engine torque multiplied by final drive ratio minus friction losses and converting from that.

Thanks, Dimman.

I know this will sound like a very noob question and Im sorry if it does, but do you think the manual will be short throws? like I don't need to mod it with a Short shifter?

Oh hey Deslock. Nice job on the calculations, I'll run some of mine here if you don't mind. Given the wheel RPM is known, 844 RPM @ 60MPH, I figure this car would need a 3.75 or so Final 5th gear ratio to pull 141MPH. That puts 6th gear around 0.78-0.80 if the 2500RPM at 60mph is true. Not exactly tall but it's not nearly as short as the MX-5/RX-8 gearbox.

Yes. It's been mentioned somewhere. I can't remember where though, sorry.

I happened to re-watch the video and noticed that 3rd gear is also shown, and estimated that it redlines at ~88 MPH (rather than ~86 as I initially guessed). So I added that information and revised my estimates assuming that 4th, 5th, and 6th are all *slightly* taller as well. Again, this is all guesswork.

top speed will be more due to the fact its got a good drag raito

my car has less hp less tq and its drag ratio is .32 and i can hit 150

more tq and hp with a drag ratio of .27 should hit 150 prob topping out after 2-3 miles at 160-165

Suppose you can hit 61 at the top of second. This is interesting since they say the car was not designed for spec sheets. If your estimates are true it would make the 0-60 time look good on paper. Perhaps sub 6 sec to 60 mph if you beat the hell out of it and are a very very good driver.

All of the gearing is a bit tall IMHO. I like it but I dunno if it's right on. The top speed gearing in 5th is around 3.7 which is on the tall side.

But 100km/h is 62mph, so for European and whatever reviews you'd be a shift short right? :O

Those ratios don't seem very close, about 1.44 between gears :O by contrast an STi transmission is 1.386 (looking at 4th and 5th at least, assuming equal "spacing"). I think first is going to be a little more than 40 since that would put it more in line with the 2nd-3rd ratio.

I think having all 6 gears equally spaced is kinda stupid if 6th gear cannot reach top speed, but that's just me (and Porsche?).

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. >:(

Anyone know where to find the ratios for an IS250 6M box? Could be a decent starting point and then we can sort out final drive.

And I've noticed in more than one review now they've talked about different gear ratios? Could there be an optional rear diff maybe? I believe the domestics still offer 'digger' low (big number) FD's on some of their products. And Toyota has offered different ratios on different spec in the past (Mk3 Supra had at least 4 different rear diffs available).

where is this video? can anyone point me to it?

Again they're only guesses, and 4th-6th are tall based on the video showing a somewhat tall 3rd (I initially had 3rd-6th all shorter before I noticed 3rd was on the video).


I previously read that it would top out at 140 (presumably due to a governor). I've since read top speed reports of 140 again, 143, and 136 (that last number supposedly due to fuel cutoff, which doesn't make sense to me).


I'm sure that's part of it, but to be fair they said they interviewed enthusiasts at track days and autox, who may have asked to be able to go above 60 MPH in 2nd (can be advantageous for autox).


FWIW, I based my guesses on cars like the RSX-S, which (according to what I read - please correct me if these numbers are wrong) hits redline at 40, 62, 86, 114, 142, 178 MPH. Because the video shows 3rd's redline speed as being ~1.5% higher than the RSX-S', I set 4th-6th to also be ~1.5% higher than the RSX-S'.

Note that the RSX-S's gearing is still taller (since it has a higher redline). Also the FT's jump from 2nd to 3rd is a bit higher than the RSX-S' (~42%-45% vs ~40%).


It is? Why wouldn't you upshift to 6th? (this car should make enough power after upshifting to continue accelerating)

If the engineers were using 5th to determine the reported top speed of 220km/hr, that actually explains the discrepancy (and would mean 5th tops out at 136 MPH). Yesterday, I asked @EdLoh about that:

@EdLoh Hope everything is OK in Japan with the quake. Back to the BRZ, is it possible they meant 220 km/hr is its top speed in 5th gear?

@deslock I'll ask and then you'll be the first to know. Likely that my Twitter dump could put the kibbosh on any more detail tho.

I added it to the original post.

Yeah, those gears are much taller than my RX8, which honestly could use a taller 6th for highway fuel economy. It goes to shit when you get above 3300 rpm, which you do when you get above around 63 mph.

Because the rpm drop of the overdrive gear(s) usually puts top gear rpm/hp too low to fight the aero drag at that point.

Top gear would be top speed provided you have the power to fight drag. FFIW the last Celica's top speed was redline-limited in 6th, I believe.

Oh you're probably right then, about the 1st gear. I guess you'd want a shorter 1st to make starts easier. I think for performance cars they either need to add another gear or have an "overdrive" unit or something, as I think you really need 6 to keep power at a good place. I applaud Porsche for their 7 speed manual :)

According to the spreadsheets at http://www.rx7club.com/showthread.php?t=541682, the FT has plenty of power to overcome air and rolling drag at 136-144 MPH after upshifting to 6th. I guesstimated 5800 RPM/135 HP at the wheels after the upshift, CoD = 0.27, frontal area approx 19.5-20.5 ft^2, sea level, and 65 F.

Even if you drop down to a slightly lower RPM, or if those spreadsheets aren't quite accurate (because they ignore increased losses due to internal friction or other factors), it seems there is enough of a margin to continue accelerating in 6th.

However, I have no experience calculating top speed and I've not driven more than 135 MPH, so I'm far from an authority on this. And I don't care about top speed anyway... I'm just curious because of the comments that it's limited by redline rather than the computer (perhaps it was a mistranslation).

It will almost certainly do 62 in 2nd. Everywhere but the US and Canada now use 0-100km/h (0-62mph) as the benchmark so I would be very surprised if they made the car need a gearchange. Even the UK no almost entirely uses 0-62 (well the manufacturers do anyway)

Has it been confirmed that the red line is the same in all gears as it is quite common to have a higher redline in 1st and 2nd now?

Edit: EVO confirms that it does 62 in second:
"Taken all the way to the redline the BRZ should be capable of dispatching 0-60mph in just under second (typo seven?) seconds (I guessed 6.7 and my chaperone just smiled), but it feels like the gearing has been very carefully chosen for the benchmark – the manual transmission (just) allows 100km/h in second gear. Actually getting a sub-7 0-60 would mean interfacing with the rev limiter a couple of tenths afterwards."

I've assumed since seeing the video's 7300 RPM upshift at 97 km/hr that you'd be able to hit 100 km/hr before fuel cutoff (though at least one report puts that at 7450).

I did a quick scan of the RX7 post and didn't see anything on wheel or tire friction losses.

I guess it will depend on how much of a fuel-economy gear they want 6th to be.

I did a gearing spreadsheet using the GenCoupe V6's transmission gear ratios (all I had handy) and a 4.10:1 final drive (GenCoupe's is 3.54:1) giving the 86 a theoretical top speed of ~167mph in 6th with 215/45R17 tires and the 5-6 shift putting the car at 5800rpm. Redline top speed in 5th would be ~132mph. From that it looks like it could break 132mph in 6th, especially if it's as slippery drag-wise as they claim.

(More real data on this car would be nice...)

Gear Ratios From GT5
 

From the GT5 game the GT-86 has the following gear ratios:
3.626
2.188
1.541
1.213
1.000
0.767
The Differential ratio is listed as 4.1 which would give the following redline speeds with 215/45R17 tires:
35.6
59.0
83.8
106.4
129.1
168.3

The video in the first post implies that for the 2nd and 3rd gear ratios above the differential ratio should really be about 3.92.
Using this value results in the following redline speeds:
37.2
61.7
87.6
111.3
135.1
176.1

Thanks for the info... I'd noticed that GT5's gearing is shorter, but hadn't yet looked it up (I cut my hand and haven't been able to play GT5 since initially trying out the GT86).

It's also noteworthy that someone posted that the top speed they were able to attain in GT5 is 137 MPH.

Anyway, I hope the real-world production gearing is closer to the game rather than what's in the Subaru promo video.

Hey guys I noticed that on the S2000 for example they list primary reduction ratio for the gearbox, but on Toyota cars you can't find this...
Is primary reduction something that varies a lot? Between all AZ6 transmissions, is it likely that any gearset or primary reduction gearset would interchangeable? I'm trying to think of ways to lower the 6th speed ratio.

I'd imagine that's because there isn't one. I'm assuming that the S2000's primary reduction is at the gearset that links the input shaft to the countershaft. On most transmissions it's simply a one to one ratio and if more reduction is needed it happens at the ring/pinion. I'd assume that a 1:1 gearset with the 9000 rpm redline would have caused some bearing or shaft to spin faster than the engineers were comfortable with.

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.

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.

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...)

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

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

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

Approximate MPH at redline (7400 RPM):

Approximate peak wheel lbf*ft (6000 RPM):

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.

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.

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

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.