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Old 08-27-2009, 01:56 AM   #1
chimchimm5
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Default Pick tire size that emulates final wheel rate of 6sp tranny

I have a WRX with the stock 5sp and a set of 16x7 wheels with 205/55/16's on them. Compared to my friends 02 bugeye WRX with 6sp swap on 17x7.5 215/45/17 tires, we found that his 2nd gear (along with everything else) was better suited for the power band on the local rallyx courses.

If I wanted to emulate his final drive at the wheel but ONLY picking a different tire size (in order to cheaply get closer to his ratio to operate in the better part of the power band), what size would that be and how do you calculate it?

(The 6sp trans' 2nd gear is noticeably shorter. My rallyx instructor commented that in contrast to my friends 6sp 2nd gear, I really need something in between my 1st and 2nd gear ratios. I'm not about to swap trannys or gearsets yet, so I'd like to go with tire if possible.)
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Old 08-27-2009, 09:37 AM   #2
Luke@tirerack
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first you need to find out what the gear ratios are for both transmissions and also both differentials. You can then start to figure out the needed revolutions per mile and make you tire size selection based on that figure
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Old 08-27-2009, 03:34 PM   #3
chimchimm5
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according to cars101, the 2006 impreza has:

5speed manual
1st- 3.454, 2nd-1.947, 3rd- 1.366, 4th- 0.972 5th- 0.738, final 3.7

So (2nd)*final = 7.2039

6speed manual
1st: 3.636, 2nd: 2.375, 3rd: 1.761, 4th:1.346, 5th:0.971, 6th: 0.756, final 3.9, reverse: 3.545

So (2nd)*final = 9.2625

-----

225/45/17 radius 12.3in circ 77.3in revs/mile 819.0

How do you figure out the desired wheel from here?

Anyways, from the ratio above, it does not look like it would be practical to compensate with just the tire size. Too great a difference.

Looks like I want a 6sp swap.

Last edited by chimchimm5; 08-27-2009 at 03:48 PM.
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Old 08-27-2009, 03:44 PM   #4
Web Foot STi
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Here it is from R&T

5spd/6spd 7.61 vs. 9.25 Overall second gear ratio.

http://www.roadandtrack.com/reviews/...mpreza_wrx.pdf

http://www.roadandtrack.com/assets/d...2003134229.pdf

Stock STi tyre is ~24" dia. For the same overall effective ratio you would need ~19.7" tyre.

Note: R&T is for a 2002 WRX.

You are even worse off: Dia=(24"/9.25)*7.2= 18.7" tyre dia!

Last edited by Web Foot STi; 08-27-2009 at 04:02 PM. Reason: typo.
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Old 08-27-2009, 03:52 PM   #5
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Engine rpm, wheel rpm, or vehical speed (MPH)?

It's all just ratios.

MPH = (rpm* Static Loaded wheel Radius" {SLR})/(overalll drive ratio*168)

Example:

MPH = (7200*12)/(9.25*168) = 55.6 mph.

Last edited by Web Foot STi; 08-27-2009 at 04:22 PM. Reason: Example
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