Confused about tire physics

According to Dunlop's site, the SP8000s 205/55R16 and 225/50R16 have the same height (diameter) and rolling radius. The only differences are larger width and larger load of the 225s. Given this, will the 225 give me a softer or firmer ride? I've heard arguments for both. There is no question that with the 225s the car will ride on more air (volume). The first argument says that air is highly compressible therefore more air in the 225s will make the ride softer. The second argument says that since each cubic unit of air will have less weight to carry in the 225, the ride will be firmer. Which one (if any) is correct? Thanks, --Leszek

The 205/55/16 diameter is 24.83".
The 225/50/16 diameter is 24.88".
If you are using the same width rim for both applications, the 225 will provide a more performance orientated ride in that you would probably run slightly higher pressures in the 225 w. a lower profile height, which should translate into a more performance orientated ride, ie: slightly stiffer.
IMO if I had a rim width of 7" or more, I'd definitely opt for the 225 width if it clears everything and on an Audi it does.
JMO, Gary.

If the diameter is virtually the same, how can then the 225/50/16s have a lower profile height than the 205/55/16s?

The profile of the tire is the ratio of sidewall height to tire width. Therefore to use a wider tire it must have a lower profile if diameter is to remain essentially constant. In the case of the tires you mention, the 205 has a 112.75mm sidewall height (205 x 0.55). The 225 has a 112.5 sidewall height. For more info, check any of the tire tech pages at TireRack, or most of the tire manufacturers' pages.

Regarding the performance question, the ride quality should be similar with either tire if the tire pressures are the same. A higher inflation pressure will give a stiffer ride. It has nothing to do directly with the volume of air.

Hope this helps.


Regards,
Dean W.