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Old 08-17-2005, 12:35 AM   #1
apak123
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Default Bought a WRX, but wrong tire sizes?

My newly aquired 04 WRX currently has new Pirelli P6 Four Season 215/55/16 on stock wheels. I looked at Pirelli's site and this size isn't in the recommended sizes to choose from. I read here that it will throw off speedometer and mileage by as much as 3% and are not recommended? Are there any other issues besides what was mentioned with this particular size? Would it be better to ditch these and go with the recommended stock sizes?
TIA.
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Old 08-17-2005, 02:22 AM   #2
nate49509
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Wow! Thats wierd. Other than the speedo being slightly off and and a little better gas mileage on the highway it won't really hurt the car. Seems like if the previous owner wanted wider tires he chould've went with a 215/50-16.
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Old 08-17-2005, 07:24 AM   #3
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It will throw off the stock speedometer and odometer - performance will be degraded slightly due to the larger/heavier tire, but probably not so much that you'll notice it. How much tread is left on the tires? I'd probably just wait until they needed replacing, then get 205/55-16's for the replacements (you could also go with 225/50-16 and have the speedo/odo be right, but that's a pretty wide tire for the stock rims)
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Old 08-17-2005, 08:24 AM   #4
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Math wise, that's only about a 1/2" difference in tire diameter and will prob only be about 1-1.5 mph slower. So if your speedo says 60, you're prob goin ~61mph. That difference will increase slightly as you go faster, and decrease as you go slower.
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Old 08-17-2005, 10:30 AM   #5
Airborne_J
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This problem can be solved in about 1 minute if someone could tell me how to calculate tire diameter for let's say a 225/50/16. Here's how I'm doing it now. Converting 225mm to inches and getting 8.86. 8.86*.5 = 4.43. 16+4.43 = 20.43 overall diameter? if this is true then:

Tire Size Speedometer reading @ 60 mph Speed when Speedometer reads 60 % Change
225/50/16 60.03 59.97 -0.05%
205/55/16 60.00 60.00 0.00%
215/55/16 59.37 60.64 1.06%
215/45/17 58.93 61.09 1.81%


yay-my first post!
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Old 08-17-2005, 10:39 AM   #6
drewdogg
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This is how i do it using 225/50-16 as an example:

225 is the tire width, 50 is a ratio of the width to calc. tire height and obviously 16 is the rim diameter. All the numbers are in mm except the rim size. SO, to find tire diameter:

1. Find sidewall height - multiply first number by the second but as a percent -> 225*.50=112.5mm
2. Then convert to in -> 112.5/25.4= 4.43in
3. That is the side wall height, but there are 2 sidewalls in the diameter, so multiply by 2 and add in the rim diameter-> (4.43*2)+16=24.86in

EDIT: i guess i should have read your post closer, you already know how to do it, just forgot the second sidewall measurement.

Last edited by drewdogg; 08-17-2005 at 10:45 AM.
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Old 08-17-2005, 10:49 AM   #7
drewdogg
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There's also a thread around here somewhere that has a link where you can enter the stock tire size and the tire size you have and it will tell you the difference.
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Old 08-17-2005, 10:51 AM   #8
Airborne_J
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Ohh, so you want two sidewalls on your tire? No big deal.

Tire Size Diameter Speedo reading @ 60 mph Speed when Speedo reads 60 % Change
215/45/17 24.618 60.63 59.37 -1.04%
225/50/16 24.858 60.05 59.95 -0.08%
205/55/16 24.878 60.00 60.00 0.00%
215/55/16 25.311 58.97 61.04 1.74%

These numbers are fairly close estimates that assume the tire is properly inflated. Anyone see any other mistakes?
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Old 08-17-2005, 10:53 AM   #9
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Looks pretty good to me...

I found that link if you wanna give a try and compare...

http://www.miata.net/garage/tirecalc.html
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Old 08-17-2005, 01:20 PM   #10
apak123
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Thanks all. You guys are very helpful.
These tires were replaced by the dealer, and are practically brand new still. I guess I will just have to wait until they wear out as I put about 80+ miles a day on just commute alone. I just didn't want any side effects like the transmission problems or such.
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Old 08-17-2005, 01:27 PM   #11
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As long as all 4 tires are the same size you're OK. w/ AWD you have to make sure they're all the same size or else you'll be turning each differencial at different rates = no good for tranny.
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Old 08-18-2005, 12:30 AM   #12
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Rotate them often to keep wear even....
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