i never can understand the tire size ratio thing so im sure somebody can answer my question here. i have 295/35/18s on an 18x10.5 wheel. can i put 285/35/18 on that rim without any problem and what will it look like compared to the previous size....less sidewall...more?

somebody help?

 

Less width, less sidewall with that tire. 35 means 35% of tread width, sidewall changes as the width changes.

 

This might help

http://www.miata.net/garage/tirecalc.html

 

This is kind of a geek reply, but I recently looked at a bunch of different tire sizes that might fit on stock rims. I used the tire shop's expertise for whether a tire might fit the rim, but I was also concerned about the changes to the wheel circumference and how that would affect the speedometer and TCS.

The first number is the tread width in mm, the second is the aspect ratio (height:width), and the third is the wheel diameter. Converting all of these measurements to inches 295/35-18 will have an outside diameter of 26.1 inches, and a circumference of 82.1". 285/35-18 will have an OD of 25.85" and circumference of 81.2". (In reality, if you measured a tire mounted on a wheel, the numbers are probably a little different - but you'll see where this is going in a moment.) With traction control, the wheel sensors monitor the speed of each wheel compared to one another (when they're significantly different, the computer interprets the difference as wheel spin and will selectively apply the brakes.) Your stock Z06 rear 295/35-18s will turn 772 revolutions/mile, the smaller 285/35-18s will turn 780 rev/mile, or 1% more. I seriously doubt the calibration of the TCS is so sensitive that this will be interpreted as rear wheel spin.

And the short version is that the sidewall on the 285/35-18 is 1/8" shorter than the 295/35-18, for an overall smaller diameter of 1/4".

 

This is yet another topic that has been discussed here numerous times. Yes, a shorter rear tire will work as long as it is still taller than the front. It will however be that much closer to the point at which the computer will interpret it as wheel spin and will be a little bit more sensitive. Also if it is off by 1%, so will your speedometer.

You do not need to go to the extra steps and calculate circumference and number of revolutions per mile. Diameter alone will do it. 1% of circumference is also 1% of diameter.

 

thanks for the great answers...that helps alot. i've always been paranoid about changing anything on my tires for fear that i'll send the whole computer into a panic attack.

 

As long as you keep a fraction of difference in diameter (rear being larger, of course) you're computer will be fine.