I have a IS 350 and it says to put 225/40-18 for the front tires and 255/40-18 for the rear tires. my question is whats the difference between the two (225 and 255) tires? Does 225 and 255 have to do with the amount of tread on the tire. is it ok to put all 255/40-18 on the car? has anyone done that? wouldn't this make tire rotation easier?

 

225 and 255 are the widths of the tire, only certain wheel and tire widths will fit on the front and rear of the car

 

By def:

eg. 225/40/18

225 mm section width (wide of the tire)
40 % of section witdth for the height of sidewall
18" diag of the wheel (rim)

edit
Best free tire calculator:
http://www.miata.net/garage/tirecalc.html

 

how do you find out the tread life of a certian tire?

 

those treadwear numbers arent really standardized, the numbers only seem to be comparable if you are staying in the same manafacturer. I've seen tires where they had a high treadwear rating and they barely lasted longer than a set of tires with a significantly less treadwear rating, both tires were of different manafacturers. Its like Adidas size 10 isnt the same as Nike size 10s for shoes

 

Go to your tire's manufacturer site and look into spec sheet of the tire. Look for UTQG listings.

There is no good info to explain how exactly government and tire manufacturers to rate the UTQG. Just like 4TehNguyen said, testing output are really different from one to another.

In general, I read it as 100UTQG = 1 year useage = 10k miles (more or less). The 10k miles will be there if the tire is on average use, and that is 12k miles in 12 months time.

Low UTQG tires are usally high traction tires and tend to wear out fast, and labled as "high performance tire". UTQG rating has nothing to do with "road noise" and "comfort ride" factors. "road noise" and "comfort ride" are manufacturer trade secrets and most peeps ignore these when buying new tires.

Buying tires get complicated when looking into UTQG, traction, and comfort. The the dolloar rule: More you pay, the better you'll get!

Hope this help.

 

After reading this thread, I went and checked my car. Sure enough, on my last oil change the dealer rotated my tires. The 225/45 17 tires are in the rear and the 245/45 17 tires are in the front. What will this do to the tires?

 

No way! Dealer fudged up big time but how can that possible. They would not fit at all.

 

You're kidding right?

Take the car back and kick the service managers @#$.

 

Hopefully they didn't "make it fit." I haven't heard any rubbing or anything. Be nice if the service manager would return a call!

 

Does anyone know if you can increase the front and rear tires to 45 or 50 instead of 40.

225/40-18 F
255/40-18 R

to

225/45-18 F
255/45-18 R

or

225/50-18 F
255/50-18 R

Is this possible? I was thinking it would give the car a smoother/cushy ride since there is more space between the rim and the road. It could also fill in the wheel well.

 

That actually happened to me. The techs didnt realize (or so they said). Go to a friends house and take your front tires off and switch them with the rear tires so that the 225's are in the front and the 245/255 are in the rear or go back tot he dealership and have them do it. When you/they do this look at your front control arms. If the control arm has an extreme shine from the tire rubing then the tire is prolly going bald on that one side almsot like u had some hardcore. Although its not permanent damage or anything dangerous u can claim that it makes u feel unsafe and get it replaced free of charge since it was their fault

My tires rubbed only when i made a hard turn. I didnt even have to be going fast. just turning sharp made this distinct noise. Get it changed asap.