The TCS operates using the difference between the front and rear outer diameters of the tires. If that difference varies more than 5 percent from stock, the TCS will not function properly. There are other things that can affect that difference - for example, the difference in tread depth between a new tire and a tire that needs to be replaced (treadwear indicator bars flat across) results in a difference in outer diameter of around 2 percent. For this reason, I generally recommend that the front to rear difference be kept within 3.5 percent of stock, so that there are no problems even as the tires wear at one end or the other.
There are two different ways to use this percentage figure to look at changes to the tire sizes to determine whether the TCS will function properly.
Method 1: Percentage difference at each end
One way is to take the percentage differences from the stock sizes at each end, and subtract them. For example, if you use front tires that are 3 percent larger than stock, and rear tires that are 3 percent smaller than stock, you are changing the ratio of front-to-rear outer diameters by 6 percent, and the TCS will not work.
Method 2: Ratio of rear to front
The other way is to calculate the ratio of the front diameter to the rear diameter. If the stock rear size is 7.7 percent larger than the stock front size, and you want to keep that ratio within 3.5 percent of stock, then you need to choose a rear size whose outer diameter is 4.2-11.2 percent larger than stock.
Either method will work.
Now, let's take the second question first.
Zennsx said:
Nxtasy,I've entered my stock 15-16" wheels into the tire calculator,(205/50/15)-(225/50/16)the difference is 7.7%. Cant it be said that 7.7% is optimal?
I hesitate to use the word "optimal" because that implies that you must hit that number on the nose. As noted above, any difference from 4.2 to 11.2 percent should allow the TCS to work. Depending on what you are doing with your tires and wheels, there may be some sizes whose ratio is not 7.7 percent, but which falls within that range. For example, lots of folks get 17"/18" wheels, with 215/40-17 and 265/35-18 tires. The ratio for those tire sizes is 6.4 percent. It's not 7.7 percent, but it's about as optimal as you can get with those wheel sizes. Hope that makes sense (and I'm sorry if it sounds like I'm nitpicking about wording, as that is not my intent).
Also note that the 7.7 percent figure only applies to the '91-93 NSX. The ratio of the stock rear to front outer diameters is 4.6 percent for the '94-01 and 5.3 percent for the '02-05. This explains why you need to do the calculations for your particular year NSX; there are some tire sizes which will cause problems for one model year NSX but not for another year.
drew said:
I still need a bit of help on my tires sizes with 1991 with OEM 16/17's (not 15/16's) and I'd like the TCS to behave properly.
I also need taller tires on my OEM 16/17's, so I can better avoid the rocks that litter the roadway during the frequent rains.
The first question I would ask is whether your car's suspension has been lowered. If you are not sure, there is a procedure in the service manual for measuring the car's ride height (page 18-6 of the 1991 manual, which is shown on page 651 of the .pdf file, which you can find
here; I'm not sure if the measurement noted there is the same for '94-05 NSXs, so those with later cars might want to check their own year's service manual). If your car has been lowered, then I would recommend raising it back to the stock ride height, rather than using oversized tires to gain ride height. If it has been lowered with Bilstein shocks on the lowering perch, you can re-install them on the other perch for the stock ride height. If it has been lowered with aftermarket springs, you should be able to find used stock springs cheap in the marketplace forums, from folks who have replaced theirs with aftermarket (you may even be able to arrange a swap with someone).
Beyond that, I have no specific recommendations regarding oversized tires to raise the car's ride height higher than stock. In the front of the NSX, in particular, tires with an outer diameter larger than stock tend to rub in the wheel well liners. Also, a ride height higher than stock may have a harmful effect to the car's aerodynamics at higher speeds. So those are concerns that you would need to consider before doing so. But as noted above, I would check the car's ride height first, as it would be better to raise a lowered car back to a stock ride height using the suspension rather than the tire sizes.
HTH
