Size check
Put your stock tire on the left and the car tire you're thinking about on the right. The math is the same for both: rim diameter plus two sidewalls, where each sidewall is the width times the aspect ratio.
Nominal sizes only. Real tires vary by brand, and a tire grows slightly once it's inflated and loaded. Width here is section width, not tread width. A size with no aspect ratio, like 175R16, is figured at 82 percent, the usual standard. Wheel rim width changes the real fit too.
What the numbers mean on a bike
Taller tire
Lower RPM at highway speed and a slower-reading speedometer if it reads off the rear wheel. Less room under the fender. On a Valkyrie, riders say a 205/65 likely needs the nut cage mod.
Shorter tire
Slightly higher RPM and a faster-reading speedometer. Sits a hair lower. More room up top.
Wider tire
Swingarm and final drive clearance gets tight fast. A GL1500 rider reports about 1/8 inch on the drive side with a 205/60.
Run-flat sidewall
Stiffer sidewalls are why many Wing riders pick run-flats. They're also harder to mount and seat on a motorcycle rim.