Think all tires of the same size ARE the same size?
 

Think again.

I just put some Yokohama Advan AD08 tires on my BRZ and I went with a 235/40-17 size.

Here are some comparison pics:

This is the new tire on an 8" wheel compared to the old one on the stock rims.
http://farm9.staticflickr.com/8142/7...2544ebfd_c.jpg
Looks bigger, but you would expect that. One's an OEM touring tire, the other is an aftermarket Extreme Performance tire made for AutoX.

But look at this comparison:
http://farm9.staticflickr.com/8164/7...5134c41b_c.jpg
Again, new tire on 8" wheel on the right. The left tire is also on an 8" wheel, but it's a 245/40-18 Continental ExtremeContact DW (which Tire Rack doesn't seem to carry any more. :iono:) I knew something was up when I noticed my new tires had more pinch than the 245s I had for my Legacy.

The 235 AD08 is listed as a 9.5" sec width, and the 245 ECDW as a 9.8" sec width at Tire Rack, but they're nearly identical in width when mounted to the same width wheel. So if you're going to be pushing the limits of what will fit, always double-check the tire width. If you go with a really high performance tire, be aware that the tire manufacturer might have fudged some numbers to get as much width as possible for the size.

How about the effects of the overall diameter? I believe the stock ones are 24.6,, would be the same with the 245/40 AD08 also at 24.6, but your 235/40 is at 24.4 (specs at tire rack). Going to a -.2 overall diameter, what would be the side effect?

New tires with labels on them..ugh I miss summer weather already. Next season can't come soon enough!

1% speedometer and odometer error, which is inconsequential, as most cars come from the factory with that much error or more. You can have that much difference in diameter going from a full tread 10/32" tire wearing it down to 3/32". I've had tires that varied as much as 5% in diameter from stock in the past, but try not to go more than that.

Each tire has a specific rim width range on which the tire can be mounted. The industry rule of thumb is that for every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10". Some vehicle manufacturers and tire companies have permitted rim widths that are not within the tire's original approved rim width range. ie.... BMW M3 & M5 and Chevrolet with almost all the Corvettes from 65 up. This effects height also thus effecting fittment.

This is a calculation I used when I was a Team TA member
Divide Width by 25.4 (25.4 = 1')

Multiply that sum by (aspect ratio = side wall height)

Multiply that total by 2 ( top of tire + bottom of tire)

Add that total to Rim diameter

Example:
235/40r-17

235 / 25.4=9.25 Section width from sidewall to sidewall

9.25 * 40% = 3.70 Aspect Ratio ie Sidewall Height

3.70 + 3.70 =7.4 Top sidewall height & Bottom Sidewall height

7.40+17=24.40 Height of tire

http://www.carswiki.net/wp-content/u...lator-pics.jpg

Typically tire manufacturers are allowed a % variance on a given size.
Many of the track-geared tires will utilize the maximum allowed variance for grip, and they are more flat to allow for more tire contact patch.

We did some similar comparisons too a while back.
265's - big differences!
http://www.tougefactory.com/evox/evox9.jpg