Ghost
07-30-2007, 03:13 PM
The computer (not sure if it's the ECU or the ABS computer) has been programmed to tolerate a variance in rotational speeds between the front and rear tires (it knows each tires rotational speed because of the ABS sensors). It knows that with the car going in a straight line, the fronts and rears should be rotating at a slightly different speed from one another based on their slightly different circumference (the difference on the stock Rays wheels wearing OEM tires sizes is +2.7% (rears are 2.7% bigger than the fronts)). The reports are that the ECU can tolerate about a 3% change from this (in other words, down to roughly 0% f/r delta and up to roughly 6% f/r delta) before it starts to think there is a problem. You have you broken the 3% rule. Here's how....
Let's look at your tires sizes in comparison to the 18 Track OEM wheels that were on my car (I use these because circumferentially, these are about the same as the stock 17's.)
We'll start with the rears first....
The tire on the stock Rays rear wheel is a 245/45-18. That tire has a circumference of 83.8 inches. The 285/30-20 you're using has a circumference of 84.0 inches, which is a difference of just 0.2%. So far so good, right? You've effectively not changed the circumference of the rear tire by going to a bigger diameter but a smaller sidewall. Now let's look at the fronts.
The tire on the stock Rays front wheel is a 225/45-18. That tire has a circumference of 81.6 inches. The 255/35-20 you purchased has a circumference of 84.9 inches, which is a difference from stock of +4.1%. Uh oh!
So, you had almost no change in circumference on the rear end when you went to the 20's, but you had a +3.3" change in circumference on the front
The end result is that there has been enough of a change between the OEM f/r variance and the new f/r variance that your computer believes a slip condition is occuring when it's actually not. Remember that with the OEM tires, the rear's were bigger than the fronts by 2.7% (we'll call this +2.7). With your 20's, the difference is that now the fronts are bigger than the rears by 1.1% (think of this as -1.1%). This is a change of 3.8%, so there's your violation of the 3% rule.
How do you fix this? Easy, drop to a 30 profile on the front tires. A 255/30-20 on the front will drop the difference from the stock tires to the new tires to just .2%, so the computer will effectively not know that there is a dfferent tire there, just like on the back. If you purchased a warranty for your tires, this might be an opportune time for the fronts to pick up some screws in non-repairable places. http://www.my350z.com/forum/images/smilies/wink.gif. Of course, I didn't say that.
Hope this helps!
Tire/wheel size calculators:
http://www.miata.net/garage/tirecalc.html
http://www.1010tires.com/TireSizeCalculator.asp
Let's look at your tires sizes in comparison to the 18 Track OEM wheels that were on my car (I use these because circumferentially, these are about the same as the stock 17's.)
We'll start with the rears first....
The tire on the stock Rays rear wheel is a 245/45-18. That tire has a circumference of 83.8 inches. The 285/30-20 you're using has a circumference of 84.0 inches, which is a difference of just 0.2%. So far so good, right? You've effectively not changed the circumference of the rear tire by going to a bigger diameter but a smaller sidewall. Now let's look at the fronts.
The tire on the stock Rays front wheel is a 225/45-18. That tire has a circumference of 81.6 inches. The 255/35-20 you purchased has a circumference of 84.9 inches, which is a difference from stock of +4.1%. Uh oh!
So, you had almost no change in circumference on the rear end when you went to the 20's, but you had a +3.3" change in circumference on the front
The end result is that there has been enough of a change between the OEM f/r variance and the new f/r variance that your computer believes a slip condition is occuring when it's actually not. Remember that with the OEM tires, the rear's were bigger than the fronts by 2.7% (we'll call this +2.7). With your 20's, the difference is that now the fronts are bigger than the rears by 1.1% (think of this as -1.1%). This is a change of 3.8%, so there's your violation of the 3% rule.
How do you fix this? Easy, drop to a 30 profile on the front tires. A 255/30-20 on the front will drop the difference from the stock tires to the new tires to just .2%, so the computer will effectively not know that there is a dfferent tire there, just like on the back. If you purchased a warranty for your tires, this might be an opportune time for the fronts to pick up some screws in non-repairable places. http://www.my350z.com/forum/images/smilies/wink.gif. Of course, I didn't say that.
Hope this helps!
Tire/wheel size calculators:
http://www.miata.net/garage/tirecalc.html
http://www.1010tires.com/TireSizeCalculator.asp