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Old 06-30-2008, 12:27 PM   #1
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Default How to make your car faster and reduce the stress on your transmission

This post is applicable in about 10 sub-forums, but I'll post it here. I've meant to do this for quite some time. It's nothing new, in fact it's quite old, but it's something that can really help now or when it comes time to shop for new tires.

How do you make your car faster while easing the load on your gears? Getting smaller diameter/height tires.

How does that help? Smaller diameter tires affect your apparent final drive. Smaller tires = lower final drive. Lower final drive = faster acceleration and decreased shock load to your gears.

How much faster will I go with smaller diameter tires? Lets look at my configuration as one example:

2004 WRX 3.90 final drive OEM tires/OEM wheels 205/55-16 24.9in tire height: OEM
2004 WRX 4.15 final drive Falken tires/OEM wheels 205/45-16 23.3in tire height: New hotness

Math for the above: Effective Ratio = (old tire height / new tire height) * original ratio

Divide 4.15 by 3.90 and you get roughly 6%. What is 6%? That equates to a 6% reduction in driveline shock and a 6% increase in acceleration.

How do I find out my tire's height? THIS LINK has a nifty calculator.

6% isn't much Well, that 6% may make the difference between your tranny busting now or later. 06/07 WRX owners REALLY should think about this as your stock 3.70 final drive puts a lot of stress on your gears. Makes you wonder why the 08s went back to 3.90 huh? 6% greater acceleration is really nice as well as this change affects you from 0 RPM to redline.

So will 6% increase in acceleration mean 6% reduction in 1/4 mile times? Probably not as there are other factors involved, but you should see some improvement. How much is up for debate.

Any negatives with smaller tires?

a. Well, when going smaller your speedometer will be off. The 6% example above means the speedometer will be 6% off on the low scale or 60MPH on the speedometer will mean 56MPH actual speed. Might save you from a speeding ticket.
b. Odometer will be 6% off on the high side as well. Might be a concern for those of you that want to do your 30K service RIGHT on the money.
c. For those that are interested in autocross or other competitive driving, smaller tires means your width and height selections are limited. Wider tires = taller tires, so there is a compromise and going from 205 width to 215 width can cost you a 1% change or more. For people that really race in one form or other or like to front with wide tires and/or 18" wheels, this post really isn't for you.
d. Spare tire concern. The stock WRX spare tire size is 135/70-16 or approximatelly 23.5" in height. You should make sure that your final tire height is approximately the same size. Stock vs. spare is approximately 3% off, so use that as a guide for you. In the above example, the spare tire BETTER matches than the stock configuration.
e. Decreased fuel economy. As with the 1/4 mile time, no one is really sure if it will be 6% or not, but there is a corresponding decrease in MPG due to higher RPMs seen at highway speed. Ficticious example: OEM tires @ 55 MPH = 2900RPM, smaller tires @ 55 MPH = 3200 RPM. But when you consider the amount your odometer will be off as above, your losses probably won't be noticed.

Supporting threads:
Wheel FAQ
Transmission FAQ
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Last edited by Unabomber; 06-30-2008 at 03:37 PM.
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Old 06-30-2008, 12:28 PM   #2
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There may be more pluses or minuses to this, but this is what I hammered out off the top of my head in a hour, so post up if you know something I forgot, got wrong, etc.
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Old 06-30-2008, 12:37 PM   #3
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Unabomber, great as usual.
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Old 06-30-2008, 12:39 PM   #4
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So are the RPMs at given cruising speed going to be 6% higher? Assuming you cruise in the 3000rpm neighbohood, that means you'll now be in the 3180-3200rpm neighborhood...I can't imagine a scenario where this would increase fuel efficiency, so maybe it would be worth noting that there is a trade off here for those that use their cars mainly for commuting. Also, I know 6% is a small figure, but increased engine speed generally equates to increased wear and tear on the bearings/internals. Over the lifespan of the car, I can imagine this being a factor to consider. Just my under-educated $.02
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Old 06-30-2008, 12:50 PM   #5
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Quote:
Originally Posted by Unabomber View Post
06/07 WRX owners REALLY should think about this as your stock 3.70 final drive puts a lot of stress on your gears. Makes you wonder why the 08s went back to 3.90 huh?
i am a little confused..i thought you said lower final drive = less stress load on the tranny? so wouldnt 3.70 be better?
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Old 06-30-2008, 12:53 PM   #6
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Quote:
Originally Posted by jcbluescooby View Post
i am a little confused..i thought you said lower final drive = less stress load on the tranny? so wouldnt 3.70 be better?
He's talking ratios not integers.
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Old 06-30-2008, 01:02 PM   #7
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hey unabomber..you think you can muster something about what psi you should run with at the track..i know hondas deflate their tires when they run but ive seen a lot of people wit AWD put more air into the tire.had a friend with a legacy who ran 14.1 stock wit his tires at 30psi and 13.9 wit his tires at 40psi
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Old 06-30-2008, 01:10 PM   #8
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Quote:
Wider tires = taller tires, so there is a compromise and going from 205 width to 215 width can cost you a 1% change or more. For people that really race in one form or other or like to front with wide tires and/or 18" wheels, this post really isn't for you.

Didn't understand that part... width and height are ideally independent if you size tires accordingly. You can have 205 and 215 tires that are identical heights.
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Old 06-30-2008, 01:16 PM   #9
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can someone explain how a taller final drive would be more harmful to the transmission? I always thought the shorter final drive would be more detrimental because it transfers more torque through the gearbox and to the ground.
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Old 06-30-2008, 01:46 PM   #10
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Quote:
Originally Posted by Unabomber View Post
b. Odometer will be 6% off on the low side as well. Might be a concern for those of you that want to do your 30K service RIGHT on the money.

Wouldnt it be 6% off on the high side? Tires have to turn 6% more to cover the same ground so it would reflect 6% more miles on the odometer.

The odo. would read 31800 after 30000 miles. (assuming tire size was changed from day one)



Also, would lighter tires/wheels have a effect?
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Old 06-30-2008, 01:53 PM   #11
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Quote:
Originally Posted by fubarud View Post
Also, would lighter tires/wheels have a effect?
Not a question of weight, just wheel slip.

Choosing a low-traction tire that breaks loose easily is nicer to your tranny than a high-grip tire of identical weight.
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Old 06-30-2008, 03:35 PM   #12
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Quote:
Originally Posted by jcbluescooby View Post
i am a little confused..i thought you said lower final drive = less stress load on the tranny? so wouldnt 3.70 be better?
3.70 puts more load on the gears than 3.90. The term "lower" in the transmission world refers to gear multiplication so 4.444 would be lower than 3.70. Seems odd, but that's what the tranny dorks use.

Quote:
Originally Posted by Timdog1650 View Post
So are the RPMs at given cruising speed going to be 6% higher? Assuming you cruise in the 3000rpm neighbohood, that means you'll now be in the 3180-3200rpm neighborhood...I can't imagine a scenario where this would increase fuel efficiency, so maybe it would be worth noting that there is a trade off here for those that use their cars mainly for commuting. Also, I know 6% is a small figure, but increased engine speed generally equates to increased wear and tear on the bearings/internals. Over the lifespan of the car, I can imagine this being a factor to consider. Just my under-educated $.02
I goofed and forgot this, edited above. Maybe I'll write down my RPMs on my way home at 55, 65, and 75 and compare notes with one of you OEM folks to see if there is in fact a 6% difference as the example above is my car. As with drag racing though, I'm not sure if the math will work out on a 1:1 ratio though.

Quote:
Originally Posted by jcbluescooby View Post
hey unabomber..you think you can muster something about what psi you should run with at the track..i know hondas deflate their tires when they run but ive seen a lot of people wit AWD put more air into the tire.had a friend with a legacy who ran 14.1 stock wit his tires at 30psi and 13.9 wit his tires at 40psi
People pump up Subaru tires for traction issues with launching vs. changing diameter. I'm not a drag racer, but as I understand it, higher PSI in stock tires for Subarus means you are more likely to get some slip which aids in launching. All the big Honda guys kinda follow what I posted and run tiny 14 or 15 inch tires.

Quote:
Originally Posted by rkramer View Post
Didn't understand that part... width and height are ideally independent if you size tires accordingly. You can have 205 and 215 tires that are identical heights.
You would think.....run the calculators and see. Wider = taller.

Quote:
Originally Posted by rougeben83 View Post
can someone explain how a taller final drive would be more harmful to the transmission? I always thought the shorter final drive would be more detrimental because it transfers more torque through the gearbox and to the ground.
Items to consider:

How many broken 3.90 WRX gearbox stories have you read? Now how many RS 4.11 gearbox stories have you read? Now how many Forrester XT 4.444 gearbox stories have you read?

A few years back Brett Middleton, the M of MRT fame, relayed a story to me of an Aussie Subaru model that would break gears left and right. The ONLY change Subaru performed that fixed the problem was swapping out the final drive for a lower one. Lower final drives put more shockload on the ring and pinion gears (beefy gears) vs. the drive gears. This saves your gears.

04/05 saw a massive reduction in broken trannys through wider gears and education on the forums. 06/07 changed to 3.70 final drive and you can't swing a dead cat in the transmission forum without a "I broke 3rd gear" story.

Quote:
Originally Posted by fubarud View Post
Wouldnt it be 6% off on the high side? Tires have to turn 6% more to cover the same ground so it would reflect 6% more miles on the odometer.

The odo. would read 31800 after 30000 miles. (assuming tire size was changed from day one)

Also, would lighter tires/wheels have a effect?
More miles is right, I got backwards thinking! Lighter wheels and tires would reduce unsprung weight, which the wheel FAQ covers.
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Old 06-30-2008, 03:43 PM   #13
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Quote:
Originally Posted by Unabomber View Post
As with drag racing though, I'm not sure if the math will work out on a 1:1 ratio though.
Agreed, I would doubt it would be a direct relationship.
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Old 06-30-2008, 04:31 PM   #14
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just get some PPG's
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Old 06-30-2008, 06:54 PM   #15
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i thought 02-05 wrx's were 3.54
and 04-05 sti's were 3.90
and 06-07 wrx's were 3.70
and 06-07 sti's were 3.54
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Old 06-30-2008, 07:28 PM   #16
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You thought wrong
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Old 06-30-2008, 07:29 PM   #17
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ok, just thought i would ask
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Old 06-30-2008, 09:21 PM   #18
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Ooh Snap!
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Old 06-30-2008, 09:27 PM   #19
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More downsides:

For anybody who cares about how the car looks, it'll look a bit ridiculous. An extra 3/4" of an inch of wheel gap is pretty noticeable.

205/40 tires provide very little in the way of protection for the wheels. I ran that setup on my mk2 gti for a year, and the tires lasted longer than the wheels did. This was a set of 16x7 OZ F1 cup wheels, street driving. There's just not much in the way of sidewall there.
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Old 07-01-2008, 02:57 AM   #20
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Quote:
People pump up Subaru tires for traction issues with launching vs. changing diameter. I'm not a drag racer, but as I understand it, higher PSI in stock tires for Subarus means you are more likely to get some slip which aids in launching. All the big Honda guys kinda follow what I posted and run tiny 14 or 15 inch tires.
Street tires do not behave the same as purpose built drag tires. Each tire will have an ideal tire pressure depending on its load weight, surface it is trying to "bite" and power applied. Add to that you have considerations about how the engine behaves with a certain launch characteristic.

In my case I ran the small Yokohama A520 tires and in my car on my track they liked about 38 psi. I got the best launch near that pressure. At lower pressures you kill your trap speed because of increased rolling resistance, at higher pressures you got excessive wheel spin and wheel hop (lots of shock load here folks --- avoid it ).

To find the right tire pressure for you situation there is no substitute for lots of drag race passes and a note book. In the beginning you are trying to solve two independent problems so it takes a lot of back and forth experimenting to sort it out if you have no starting point.

I just kept notes of all my runs and played with launch performance (60' time) and trap speed and tire pressures. Over time you find that your best runs tend to cluster near a certain tire pressure. Then you pick that tire pressure and sort out your launch technique until it is stable and repeatable. Once that is done then you go back to your tire pressure and start bumping the pressure up and down in about 2 psi increments and watch the results.

You are solving two issues that interact here. Ideal traction for best launch and proper wheel slip under your launch condtions to make the engine happy. Maximum traction occurs with about 10% - 15% slip between the tire and the surface (just like threshold braking). You want to get the right tire pressure so you get that limited slip, and at the same time keep the engine happy by keeping it in an rpm range it can pull hard in.


One other down side to short tires is ground clearance you will have to watch speed bumps and your mufflers. Yes the car looks a bit odd, but that is not an issue when folks come up to you after a run and ask you how you get such a hard launch.

It also really pisses off the Corvette and Mustang guys when you hole shot them on the launch and they can't quite make the lost ground up by the time they get to the traps .

In my case the Yokohama's were literally the shortest tire I could find on tire racks web site (205x40 A520 which is out of production) for the 16 inch wheel at 929 revs per mil vs about 830 revs per mile for the RE92 stock tires. The current equivalent tire is the Yokohama 205/40 VR16 Parada spec 2 at 929 revs per mile. This gave me an effective gear ratio of about 4.36 compared to the physical gear ratio of 3.90 or about a 12% change. Note tire size (revs per mile) varies across manufactures even for similar nominal tire sizes.

It made a major difference in the behavior of the car, and unlike physical gear changes you can do it at the track put your stock tires back on and go home with a normal appearing car.

Larry

Last edited by hotrod; 07-01-2008 at 03:07 AM.
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Old 07-01-2008, 01:20 PM   #21
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In terms of reducing stress on the transmission, is switching to a smaller tire that has much more grip going to help much? I'm guessing that sticking with the RE92's or a similar all season is more likely to save your transmission. I wonder what the probabilities are for blowing the transmission with stock vs. summer tires? I've always been amazed at how many people destroy gears. I launch my car daily, and have no issues whatsoever with my gears with 160k on the 03, running all seasons at high psi.
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Old 07-01-2008, 04:01 PM   #22
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Like someone else already said, a smaller diameter tire = more mechanical advantage to the car, but it also leads to higher cruising rpms = more wear and tear and decreased gas mileage.
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Old 07-01-2008, 04:10 PM   #23
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I used to run low pro Azenis on RS wheels. I loved them except for on the highway.
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Old 03-20-2009, 10:16 PM   #24
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To clarify something, doing this will increase acceleration OR reduce stress on the tranny, not both simultaneously. At the same acceleration the new ratio will require less power from the engine, reducing the stress through the whole powertrain. However applying the same amount of throttle (say, flooring it) will produce the same amount of power from the engine, resulting in the same amount of stress through the powertrain, which will result in the increased acceleration. Still though, two nice benifits from one simple change.
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Old 03-21-2009, 12:24 PM   #25
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I did this last year,... One of the best things I have done to the car.
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