kcobean

iTrader: (2)

I've been asked many times about altering the tire sizes on 350Zs equipped with TCS and VDC....what you can do, what you can't do, etc. So I figured I'd write this for people to use as a reference on what tire sizes they can get away with without upsetting their cars electronics. At the end of this article, there are some links to the tire size calculators that I used.


This is an actual PM I received from someone who was having problems with their car after putting new tires on his car. (Minor edits made for "printability")
############################################################ ############################################################ ##########################################

Kelly
I just bought the Axis MAtrix (f) 20x9 and (r) 20x10.5, Tires: Falken grb-fk451 255/35/20zr, 285/30/20zr, and they look and drive great. But sometimes when taking a sweeping turn or making a quick turn the traction control activates more so now then with the stock rims and tires. It seems to stay on longer and come on when there is no real reason. Does anyone else experience this or issues with traction control?

Sincerely,
Confused in treadville



Howdy confused.....put on your thinking cap...this might hurt a little

Yes!, you're having a problem as a result of your tire choice. Here's why.....

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. . 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

kcobean

iTrader: (2)

It's been a year since my original post so I thought I'd update this thread with an experience a my350z.com member recently shared with me.

This member ended up with wrong-sized tires on a VDC enabled car courtesy of an uneducated tire shop. So read on and remember, the moral of the story is that sometimes the dealers and tire shops don't know how to pick appropriate tires for your car, so you should! A little persistence saved this member from thousands of miles of frustration driving around with a computer that thinks the tires are slipping when they're not.

Just for the record, I asked for clarification of a) the car's original tire sizes, b) the "wrong-sized" tires the shop put on the first time, and c) the size they were going to put on to correct the problem.....

And the good news.....

OUTSTANDING!!! Not one, but TWO dealers advised this member to drive the car in a "crippled" configuration or turn off a major safety feature!

Don't let this happen to you! If you have VDC, do the math before you lay down hard-earned cash for tires.

See ya!

DavesZ#3

iTrader: (26)

Another good source of info on the proper choice of tires sizes can be found in this post within the Top 100 Questions thread.

CLICK HERE

Chebosto

iTrader: (43)

Another good tire wheel offset website to look at before asking "WILL IT FIT BLAH" is in fact the website..

http://www.willtheyfit.com/