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Discussion Starter · #1 ·
I need to order new tires, I have 18X9 rims. I want to get wider tires, should I get the 255/40/r18s which are .5 inches larger in diameter than stock or should I get 265/35/r18 that are nearly the same diameter as stock tires? Are 35 series tires going to make the ride a lot bumpier and more prone to dent my rims? thanks
 

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Matt Sherman
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VIN_DIESEL said:
you should get the 40mm profile tires otherwise your spedo will be off. Also , the ride will be bumpy'er with 35mm profile tires.
:stupid:

Matt
 

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chunsangp
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If you go with 265, then you should get 265/35. 255/40 18 will make your speedo off more than 265/35. I would go with 265/35.

Also that 40 or 35 thingy, it's not mm. It's some kinda ratio.
 

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THE GAME
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chunsangp said:
If you go with 265, then you should get 265/35. 255/40 18 will make your speedo off more than 265/35. I would go with 265/35.

Also that 40 or 35 thingy, it's not mm. It's some kinda ratio.
sorry but you are tottaly wrong. It is millimeters and his spedo will be off if you put a 35mm profile tire on an 18 inch rim.
 

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Actually, No, its not MM it is a ratio. Say you have 265/35/18 the wall height would be (265*.35 = 92.75MM, 92.75/25.4 = 3.65 IN)

255/40/18 would be: (255*.4 = 102MM, 102/25.4 = 4.02IN)

Here is something that will explain it:

http://www.howstuffworks.com/tire2.htm
 

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Badassical Baddage
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VIN_DIESEL said:


sorry but you are tottaly wrong. It is millimeters and his spedo will be off if you put a 35mm profile tire on an 18 inch rim.
Copper said it, it is not millimeters.

Get the 265/35 I have 255/40s right now and, while it fills up the wheel gap very nicely, it does make the speedo off a bit.
 

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Get 265 or 275's
 

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Badassical Baddage
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245 will be a little narrow on the 9" wheel. I wouldn't get anything less than 255!

Stick with the 265s or get 275s though!

275/35/18 is almost exactly the same diameter/circumfrence as the stock 245/40/18
 

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THE GAME
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copper2002 said:
Actually, No, its not MM it is a ratio. Say you have 265/35/18 the wall height would be (265*.35 = 92.75MM, 92.75/25.4 = 3.65 IN)

255/40/18 would be: (255*.4 = 102MM, 102/25.4 = 4.02IN)

Here is something that will explain it:

http://www.howstuffworks.com/tire2.htm
woops , you are right. I knew that the first number is millimeters so i just figured the second number is also millimeters. But in any case the spedo will be off if you put a 35 tire on a 18 inch rim.
 

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VIN_DIESEL said:
But in any case the spedo will be off if you put a 35 tire on a 18 inch rim.
Not really. 275/35/18 is almost exactly (.16") the same as the stock 245/40/18. I don't think you'd even be able to tell that difference on the speedo. It's probably even within the factory tolerence limits for the speedo.

Think of a worn tire, it's probably .375" smaller than stock after a couple years of driving!
 

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JagdStealth said:


Think of a worn tire, it's probably .375" smaller than stock after a couple years of driving!
ok , but then when the 35 tire wears .375" that will make a difference. Maybe you are right and it won't be a noticable difference. All i am saying is that there is a difference and i would keep the stock sidewall ratio.
 

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Then you need to keep the stock width.

if you want wider than 245, you need to change the sidewall ratio to keep it closer to the actual speed.
 

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Discussion Starter · #15 ·
alrighty after some searchin I found this very helpful http://www.miata.net/garage/tirecalc.html


Tire Size Comparison

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference

245/40-18 3.9in 12.9in 25.7in 80.8in 784 0.0%
255/40-18 4.0in 13.0in 26.0in 81.8in 775 1.2%
265/35-18 3.7in 12.7in 25.3in 79.5in 797
-1.6%
275/35-18 3.8in 12.8in 25.6in 80.4in 788
-0.5%

So if I got 255/40/18 my speedometer would only be off by 1.2%? Seems like a pretty unoticable amount and and its the optimal size for a 9" wide wheel.
 
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