Tire size and ride height

[Matt Knight]

I am in the process of replacing the Goodyear Eagles (225/45-17 front and 265/40-18 rear) that were on the car when I bought it in 2008. These tires seem fine to me (I have no basis for comparision); however I do set off the TCS under quick acceleration.

My intention is to replace them with BFGoodrich g-Force T/A KDW's (215/40-17 front and 265/35-18 rear). I do not track the car but do enjoy spirited driving in rural areas and safe conditions. These tires seemed to be a good choice for my car and how I drive it.

I am concerned with dropping the ride height as my driveway has a sharp upward angle that requires a very slow, 45 degree approach. I have about 1" of clearance (without a front lip).

Will changing the tire size drop my front end more than 1"?

Thanks in advance for the help,

Matt

 

[Hapa88]

Front tire diameter difference is as follows:
Original 225/45/17 = ~25" diameter
New 215/40/17 = ~23.8 diameter

Difference is 25-23.8 = 1.2" where you car would lower 1/2 of that difference. So expect your front end to ride 0.6" lower than before.

For the rear:
Original 265/40/18 = 26.3"
New 265/35/18 = 25.3"
Difference is 1", but again the lowering is only 1/2 of that = 0.5" lower

You can use this tool to compare two different tire sizes
http://www.rimsntires.com/specs.jsp


I have personally run your "New" tire sizes on my car with no TCS issues on my 1995.

 

[nsxtasy]

Adrian's advice is spot on.

Incidentally, with those HUGE oversized tires you're currently using, your front tires are probably rubbing at full lock; that should no longer be the case with the 215/40-17 fronts. Also, your rear tires are 5 to 6 percent larger than the stock size, so your speedometer has been understating your speed by that percentage (actual speed is 5 to 6 percent higher than what the speedometer says, assuming no other variations); that will no longer be the case with the 265/35-18 rears, whose outer diameter is less than 2 percent different from stock.

 

[Matt Knight]

Adrian, Ken; thanks so much for the helpful input; it's Primers like you that make this such a great forum.

As I am moving to a tire size that (for lack of a better term) is more appropriate; what changes in handling can I expect? I'm curious to know what will set off the TCS.

Might have to buy a new house if the driveway is not compatible with the new tires:biggrin:.

 

[nsxtasy]

As I am moving to a tire size that (for lack of a better term) is more appropriate; what changes in handling can I expect? I'm curious to know what will set off the TCS.

You shouldn't expect changes in handling. Handling tends to be a function of the width of the tires, more so than the outer diameter. Moving from 225 and 265 to 215 and 265 shouldn't have any significant effect. If anything, the shorter sidewalls of the new normal-sized tires should result in a slight improvement in cornering, at the expense of slightly harsher ride comfort, but those changes should be small.

The TCS has nothing to do with the width of the tires. It operates as a function of the difference in outer diameters, front vs rear. It should be fine with the new sizes, although it probably should have been fine with the current oversized sizes too. What year is your NSX?

 

[Matt Knight]

Ken,

It's a 1991, with 124K miles and stock suspension.

When I test drove the car the TCS light went off when I accelerated; the PO said it was due to the aftermarket wheels and tires being different sizes than the 16/17 OEM setup.

I've had no rubbing issues with the current tires.

I appreciate the education,

Matt

 

[nsxtasy]

When I test drove the car the TCS light went off when I accelerated; the PO said it was due to the aftermarket wheels and tires being different sizes than the 16/17 OEM setup.

That's odd. The TCS works based on the ratio of the outer diameter of the front tire to that of the rear tire. A rule of thumb is that the ratio needs to stay within 5 percent of stock for the TCS to work properly, but other variables can come into play (for example, the difference in outer diameter between a new tire and the same tire worn down to the treadwear bars is about 2 percent). The outer diameter of the 225/50-16 stock rear tire on a '91 is 7.8 percent larger than that of the 205/50-15 stock front tire, so theoretically, TCS would probably work with rears 2.8-12.8 percent larger than fronts, especially if they are closer to that 7.8 percent figure than the endpoints of that range. 265/40-18 is 5.5 percent larger in diameter than 225/45-17, so I would have expected no problems with the TCS, unless perhaps the front tires are practically new and the rear tires are quite worn. Oh well, the new sizes should definitely not cause problems with the TCS, and if they do, then there is some other cause.

 

[Matt Knight]

Thanks again to Adrian and Ken for their help. You know your an NSX guy when... buying tires feels like a major life decision:biggrin:.

Got the new tires today, no TCS issues and was able to clear the curb getting into my driveway. Life is good.

 

[Hapa88]

Great news! :smile: