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.

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

I figured this thread would've died long ago when the ratios were revealed.

143 MPH makes more sense as a top speed than the report of it topping out in 5th (as there really ought to be enough power in 6th to get above 140).

The calculated tire diameter is 24.6", but it's less when loaded down and changes with speed. If 5th tops out at 131, and if you need to upshift to 3rd before 60, the actual diameter would be around 24.3" (though it might be a tad smaller at 60 MPH and higher at 131 MPH). I used 24.31" for this table:

I don't think tires lose diameter as they're loaded, because the very outer strip of rubber is what's rolling on the ground and it's not going to deform much, but I think you can't go off tire width * aspect ratio + rim diameter, apparently there are charts with the correct rolling diameters of each tire size but I haven't seen them before. 24.3 is what I heard was the correct diameter though.

This is kind of interesting, it appears to hit top speed around its 6th gear torque peak. Still, I'd rather give up 10mph top speed for better fuel economy on the highway.

If you go to a place like tirerack and look up the tire they give the RPM @60mph loaded. I've found it to be around 3% more RPMs than calculated.

Deslock, can you output numbers or did you do that by hand? I'd like to see auto vs manual and even compare with 3.727 gears if you could do that easier than me.

redline is at 7450 rpms so 60mph is possible!

That's still unknown. I picked those numbers as a middle ground between Insideline's report that the rev limiter in 2nd kicks in at 59.2, and serialk11r's comment that 5th maxes out at ~131.

In retrospect, serialk11r probably based 5th maxing out at 131 on my comment earlier in the thread (note in that in that same comment, I had written that it hits 60 MPH in 2nd). Tire deformation is such that it'll have a higher rolling radius at 131 MPH than at 59.2 MPH, but I don't know the formula.

Maybe 2nd does top out at 59.2 MPH, or maybe Insideline's tires were slightly deflated, or maybe Insideline's results were slightly off due to a mishap with their testing equipment. Who knows? I started this thread because I was curious about gear ratios in general, but I don't care if 2nd tops out at 59.2 or 60.8 MPH. But FWIW, GT5 lets me get to 60 in 2nd gear :D


Since we don't know the story regarding rolling radius, here are the two extremes for all three transmissions (again how tire deformation varies with speed is not accounted for):

6MT, DR 3.727, Dia 23.975"
6MT, DR 3.727, Dia 24.6"
6MT, DR 4.1, Dia 23.975"
6MT, DR 4.1, Dia 24.6"
6AT, Dia 23.975"
6AT, Dia 24.6"

Going off of the tirerack specs for a similar tire, the RPM@60 is 844. Tire circumference is about 24.7"

Strangely the actual stock tires used don't have these specs, probably not that far off though.

How did you make those BTW?

Actually, I just google searched, and what do you know, the first link is to ft86club.com...
-_-

Apparently the correct rolling circumference for a 215/45/17 tire is about 23.9 inches, so I guess there wasn't a mistake.
Funny thing is, 205/55/16s probably hit 60 in 2nd lol. Another reason to want the 16" rims, not only do they look better and tires cost less, but you can brag about a faster 0-60!

The rolling circumference isn't going to change from load. The tire may look deformed, but the outer strip of rubber that is the tread will not have changed overall length. However I'm not sure what's going on because no matter how section width is measured the aspect ratio is going off distance from rim to tread...the person who said 23.9 inches got it from an official European tire regulatory thingy website, and it seems to be in line with the observation that 2nd gear doesn't hit 60mph. Or maybe their GPS tracking was too slow, but if they use it on other cars just fine doesn't seem like it would be the case.

The measurements for a 215/45/17 tire just by doing the math is 24.618" so there's no way it's 23.9"

Check my math.

spreadsheet

60 * tire_circ_feet * RPM / (gear_ratio * DR * 5280)

Being in Canada I am more used to KM/H rather than inch/feet/mile. I also use spreadsheet to do these calculation, and my results are closed to what Deslock provide.

Some points;

1. Tire diameter for 215/45/17 is 625.30 mm / 24.62 inch *without* considering the deformation. I do not know how that is determined but a 0.7" deformation could be significant.
2. The default tire selection is 205/55/16 with diameter 631.90 mm / 24.88 inch. So if you swap this tire with the 17" one from OEM you could reach 60 mph in 2nd gear with larger circumference.
3. Based on its gear ratio, the automatic should be able to hit 60 mph in second gear. That could mean a faster 0-60 time.
4. According to the Subaru website in Japan, if the care comes with 16 in wheel, its final drive ratio is *different*. I wonder if they have to use a different final drive ratio to compensate the difference of circumference between the 16" and 17" wheels.

That's it for now.

Another note: does anyone knows why the top speed is at 145 mph? Was that government limited or simply not enough power to go higher? Based on the spreadsheet, with 3.727 final drive ratio, one could reach 300 KM/hr at 7200 RPM. This could be a contender in the Tokyo Midnight Wangan run.

No way would it hit 300 km/hr. Most of my tables don't reflect top speed, which is reportedly drag limited.

There's got to be some tire deformation, but 0.7" is higher than I'd expect for 215/45R-17 (maybe I'm wrong).

You brought up a good point regarding the 16" rims... if those are standard on the cars with a 3.727 DR, then (ignoring deformation):

6MT, DR 3.727, Dia 24.88" (205/55R-16):
I had intended to clean up my spreadsheet and post it on google docs, but never got around to it.

Oh yeah. <.> *derp*

Where did I put my gear ratio spreadsheet...

Tire deformation is going to be very small if you think about it. The rolling circumference of the tire is the length of the tread. The tread itself is reinforced with steel underneath, so only the surface can experience minor stretching. The rubber part of the tire is well under an inch thick.

So the question is, why does some official tire spec regulatory body in Europe say a 215/45/17 tire is 23.9" when the way tires are measured implies it's 24.6"? Why is the car not hitting 60mph in 2nd gear if the tire is indeed 24.6"?

For the first part, do you have any link or page where it said it is 23.9"? There might be a different standard used in Europe, or the P type wheel (P215/45/R17). I am guessing this might be the case.

Second part, Based on the spreadsheet, in order to hit 60 MPH mark on 2nd gear, some conditions need to be met:

1. the tire circumference is indeed based on the ideal value of 24.62" diameter.
2. the tester actually revved it all the way pass 7300 rpm or be hugging 7450 before up-shift. I think this is not the case where insideline test their sample of BRZ.
3. the other tester who can clock faster 0-60 time may also be using the automatic, as seen in the spreadsheet that it can reach 60 MPH sooner at 7000 RPM.

Too much reliance on math... Section/aspect ratio is an approximation given different companies manufacturing. They have specific measured dia/circ numbers for specific tires and they don't all match the formula numbers.

[QUOTE=Want.FR-S;171844]For the first part, do you have any link or page where it said it is 23.9"? There might be a different standard used in Europe, or the P type wheel (P215/45/R17). I am guessing this might be the case.

The reduced diameter is not from squish. It is from the difference between official tire specifications and the rough number calculated from the wheel diameter plus the nominal aspect ratio times width. The 1906mm rolling circumference is from official ETRTO tire specs. (I don't expect that tire size standards vary between Europe and Japan and the US. From the wikipedia ETRTO entry it just looks like Europe standardized tire sizes first.) Here is a source for ETRTO standard tire circumference:
http://www.club80-90syncro.co.uk/Syn...calculator.htm
Here is a calculator for circumference including increase in circumference with speed:
http://bndtechsource.ucoz.com/index/...alculator/0-20
Here is a Yokohama tire size chart where you see a circumference of 1906mm for the 215/45R17 tires:
http://www2.yokohama-online.com/gb/t...php?page=23084
So, until someone posts a better link to tire circumference info, I think the 215/45R17 tire has a 23.9" effective diameter.

[Edit] OK, reading Tirerack's explanation maybe the difference is due to "squish":
http://www.tirerack.com/tires/tirete...jsp?techid=203
Looking up the Primacy HP, Tirerack doesn't list a revs per mile number. But, they do have one for the Michelin Pilot Sport 3 in a 215/45 R17 size, and the value is 844. This is a 23.9" equivalent rolling diameter so I still think we should use 23.9" for calculating wheel RPM vs speed.

Cool, thanks for the links. With this said it is more accurate to calculate the numbers based on the actual circumference rather than wheel width/ratio/diameter. With that revised number, one *cannot* hit 60mph (59.04) with Toyo's 215/45/R17 tire.

Just check Michelin's website, the Primacy HP 215/45/R17 has 844 rev per mile. with 24.7" as the overall diameter. Convert that using one mile - 1609.344 m, it comes to 1.9068 m per rev. So we should stick the circumference number for calculation purpose. :)

I would think there is no need to reverse calculate the *effective diameter* as this may confuse people and trying to reason it with deformation or other effect. In fact, if you know the rev number per mile/km, it is easier to calculate the circumference more precisely. Otherwise, how one could explain the difference between 24.7" and 23.9"?

Revs per mile by Michelinman.com for Primacy HP 215/45/17 is 844.

Yes, I agree we should just use the revs per mile or circumference value. I am not sure what the 24.7" number really means: Maybe the tire bows upward in the middle when unloaded.

You guys are forgetting the obvious; we're working with pneumatic tires. The rolling radius is entirely dependent on inflation pressure and loading weight. The circumference is always the same, but since the vehicles weight is resting on the tire, the tire itself forms a "D" shape when viewed from the side. Tire Rack's listing of "Revs per mile" is the only useful statistic if you're trying to calculate speed from a known axle RPM. Of course, that measurement is entirely assuming a typical inflation pressure and loading weight. The "overall diameter" has nothing to do with the tire mounted on your car and sitting on the ground.

Deslock's chart's using an approximate 24" rolling diameter are appropriate for the stock 215/45-17.

Exact 6MT speeds with 215/45R17 tires
 

Since I just calculated the exact 6MT gear ratios I can post the exact speed vs RPM for the standard 215/45R17 Tires using the known ETRTO spec of revs per mile for the tires which is 844.4:

RPM- : 1st- | 2nd- | 3rd- | 4th- | 5th- | 6th-
-------------------------------------------------------
1000 : +4.8 | +7.9 | 11.2 | 14.3 | 17.3 | 22.6
1250 : +6.0 | +9.9 | 14.1 | 17.9 | 21.7 | 28.2
1500 : +7.2 | 11.9 | 16.9 | 21.4 | 26.0 | 33.9
1750 : +8.4 | 13.9 | 19.7 | 25.0 | 30.3 | 39.5
2000 : +9.6 | 15.8 | 22.5 | 28.6 | 34.7 | 45.2
2250 : 10.8 | 17.8 | 25.3 | 32.1 | 39.0 | 50.8
2500 : 11.9 | 19.8 | 28.1 | 35.7 | 43.3 | 56.5
2750 : 13.1 | 21.8 | 30.9 | 39.3 | 47.7 | 62.1
3000 : 14.3 | 23.8 | 33.7 | 42.9 | 52.0 | 67.8
3250 : 15.5 | 25.7 | 36.5 | 46.4 | 56.3 | 73.4
3500 : 16.7 | 27.7 | 39.4 | 50.0 | 60.7 | 79.0
3750 : 17.9 | 29.7 | 42.2 | 53.6 | 65.0 | 84.7
4000 : 19.1 | 31.7 | 45.0 | 57.1 | 69.3 | 90.3
4250 : 20.3 | 33.7 | 47.8 | 60.7 | 73.7 | 96.0
4500 : 21.5 | 35.6 | 50.6 | 64.3 | 78.0 | 101.6
4750 : 22.7 | 37.6 | 53.4 | 67.9 | 82.3 | 107.3
5000 : 23.9 | 39.6 | 56.2 | 71.4 | 86.7 | 112.9
5250 : 25.1 | 41.6 | 59.0 | 75.0 | 91.0 | 118.6
5500 : 26.3 | 43.6 | 61.9 | 78.6 | 95.3 | 124.2
5750 : 27.5 | 45.5 | 64.7 | 82.1 | 99.7 | 129.9
6000 : 28.7 | 47.5 | 67.5 | 85.7 | 104.0 | 135.5
6250 : 29.9 | 49.5 | 70.3 | 89.3 | 108.3 | 141.1
6500 : 31.1 | 51.5 | 73.1 | 92.9 | 112.7 | 146.8
6750 : 32.3 | 53.4 | 75.9 | 96.4 | 117.0 | 152.4
7000 : 33.5 | 55.4 | 78.7 | 100.0 | 121.3 | 158.1
7250 : 34.7 | 57.4 | 81.5 | 103.6 | 125.7 | 163.7
7450 : 35.6 | 59.0 | 83.8 | 106.4 | 129.1 | 168.2