Well after a lengthy discussion with a guy at a dealership (not the typical crook) today. I have finally figured out the numbers on tires. Since we are in the good old USA, I prefer inches over millimeters.
While researching info for the upcoming CBR swap on Susan's bike. I wanted to get a tire that was close to the same overall diameter as the stock tire. But the stock wheel is a 15" and the CBR is a 17" So after much scratching of the head and almost burning out my calculator I have some info.
The stock rear tire on the SS 750 is a 160/80-15
The 160 represents the width in millimeters
The 80 means that the sidewall is 80% of the width
The 15 is the rim size
So, if you take the 160 and multiply it by .80 that will give you the sidewall height in mm's. Then take that number and multiply it by .03937 to convert it to inches. Take that result and multply it by 2 (sidewall on top and bottom of tire) and ten add the diameter of the rim, in this case 15"
This will give a pretty close overall measurement of the entire wheel.
Let's review: 160 x .80=128mm
128 x .03937=5.03 inches
5.03 x 2=10.07 inches
10.07 + 15=25.07 inches total height
So now I want a wider tire that will fit on the CBR 17" rim and still be near the stock overall diameter. What I found was a 200/50-17. The overall diameter of this will end up being approx. 24.87" which is just under the stock tire diameter.
The front tire is a whole different challenge. The stock front is 19" and the CBR is 17" I need to find a tire that will fit the smaller rim but have a taller sidewall that won't leave a huge gap between the tire and fender.
The stock front is a 110/80-19 with an overall diameter of 25.92 inches
The closest I've found in total diameter is a 120/90-17 which has an overall diameter of 25.5 inches. But the problem with this is that the tire is 10mm wider and that presents a clearance issue to the fender mounts. So the next available size would be a 110/90-17 which has a diameter of 24.79" which would make the gap between the fender and tire a half inch wider. That I can live with.
Just for quick reference these are the inch equivalants for rear tire widths
owwww....my head hurts