Tire Diameter?

This may be a little off topic, but it pertains to tow vehicles and boat trailers. And, this may be really stupid, but...When you change wheel sizes it seems that everyone likewise changes the profile of the tires they use. For example, when putting on 20" wheels from 17" or 18", it looks like you go to lower profile tires. Is that done in order to keep the tire diameter the same? If you didn't do that, I would expect that the tire diameter would be larger than original equipment and either you would have to figure out a way to recalibrate your speedo and odometer or have inaccurate readings on both. Can anyone explain the whole process. I am thinkin about getting larger wheels on my 2006 Jeep Grand Cherokee Limited Hemi but haven't done any research on it yet. Thanks.

You are exactly right. I found some articles that have more information. If you don't follow that model you would need to adjust your suspension to fit the larger diameter.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=25
http://www.pepboys.com/learning_center/car_care/articles/tire_plus_sizing.html

Here is how to calculate the size from Tirerack's pages:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=7

You can sometimes find replacement drive gears for the speedo that fit internally in the tranny to compensate for up or down sizing of the diameter.
One thing that many people don't take into consideration is the torque change that you get when you up-size the diameter. You are changing the overall drive ratio with a larger diameter tire just as if you were going to a higher pitch prop or changing the gear ratio on your vehicle.
The result of a larger tire diameter is slower acceleration and a lower cruising RPM.

You also take away some of the aerodynamics and limit yourself to fewer tires but these are of lesser impact.

http://www.miata.net/garage/tirecalc.html

the easiest one I've found.