Re: **UPDATE 12/16/09**Replacement Tires...here is some info.
After reading all of the info on suspension and tire sizes with speedometer error. I am thinking to change my 235/50-18 for 225/60-18. Would they give a softer ride? Also if I am reading the tire size chart correctly would they give a more correct speedometer reading?
Rich
There are plenty of stories of guys ripping up fender wells or shredding tires that are too big for their cars, so be careful when upsizing. The safest method is to ensure the outer diameter isn't larger...but width is just as important, and going smaller can also be a problem (brake interference.)
When comparing tires and wheels you want to look at diameter and offset. Here are a few good guides.
The first one yields a matrix of tires that aren't bigger than the originals.
http://ejelta.com/tiresize/index.html?ws17=1&tiresize=235/50-18&minwidth=&maxwidth=&minratio=&maxratio=&minwheel=&maxwheel=&maxdelta=2
The second is a tire and
rim size calculator that includes all the important metrics.
http://www.rims-n-tires.com/rt_specs.jsp
and finally, a real simple one that just compares diameter and revs per mile.
http://www.miata.net/garage/tirecalc.html
Using the last one, yields 740 revs per mile with the stock 27.3" dia tires and 704 revs per mile for the 28.6" 225/60 18's. That is about a 5% difference.
There is always a risk the larger diameter tires might interfere with the
wheel well, but going narrower helps. I suspect you won't have any issues with the extra 5/8's of an inch of radius.
As far as ride comfort is concerned the most important factor is sidewall stiffness. All things being equal, a larger diameter tire will have a less stiff sidewall, but tires themselves vary tremendously. For example, high performance tires will have stiffer sidewalls than standard tires. But even with one type of tire...like high performance, there is variation. Some high performance tires have much stiffer sidewalls than others.
If you really wanted to make a difference in ride you could go down in
wheel size. The 17" wheels will yield taller sidewalls, and, all things being equal, a softer ride. I put 215/60 17's on, for winter tires, and the ride is noticeably less jarring than the stock 235/50 18's. Part of this is the size change and part of this is going from stiff performance tires to softer winter tires (Michelin X-Ice.) You will trade a bit off in performance. Despite Michelin X-Ice being one of the better performing winter tires, they are noticeably more squirrelly on dry pavement when pushing the car to it's drifting limits. Of course, if you aren't the type, like me, who likes to occasionally push the car to the limits, you won't even notice.
