View Full Version : Question about the middle number in a tire size (e.g., 50 v. 55)

05-17-1999, 08:13 PM
In a tire size such as 225/50/16, the middle number is related to the "tallness" of the tire, no? So if I were to put 55s instead of 50s, the car would sit a little higher off the ground?<p>And if this is correct, what other effects does messing with this parameter effect? Speedometer accuracy would seem to be an obvious one. <p>I'm just wondering if I should opt for 55s when I replace my stock Contis with Dunlop 4000s.

05-17-1999, 08:48 PM
205/55-16 means that the height of the sidewall of the tire is 55% of the width of the tire, or 55% of 205mm. Take this number times 2 and add the wheel size and you'll get the total wheel and tire height. This also means that 225/50-16 yields almost exactly the same height.

05-18-1999, 03:29 AM
Thanks!<p>Okay then - this means (using your examples) that a 225/55-16 would be almost an inch (22mm = .87") taller overall? Is that correct?<p>225 x 55 x 2 = 247.5mm<br>205 x 55 x 2 = 225.5mm<br>----------------------<br> = 22mm or .87"<p>That's a significant height change! <p>My real question then is, why don't people seem to use this factor when attempting to raise/lower their cars (I mean just change height -- not performance issues -- pure cosmetics)? It just seems like one could get even a small adjustment to height by using a small tire size change. For example, if I wanted to raise my car 10mm (I just put on H&R springs and Bilsteins), why couldn't I just replace my stock 205/55-16 tires with 215/55-16s? That would give a height difference of 11mm, if I'm calculating and understanding this concept correctly.<p>-Brian

Dave W
05-18-1999, 06:13 AM
The reason why people don't, or shouldn't use this method to change the ride height is because you are changing the overall diameter of the wheel/tire. If you add an inch to the outside diameter, the speedometer would be off. To keep the speedo correct, the overall diameter of the wheel/tire needs to remain constant. This is where the idea of + and - sizing comes from. You can "plus 1" size a 16" wheel by going to a 17" wheel, but the tire will have a shorter profile to keep the overall diameter the same.<p>Hope this helps<br>dave<p>17" wheels

05-18-1999, 11:04 AM

05-18-1999, 04:16 PM