Tire Measurements

120/70ZR17:

"The most important label is the sizing designation, which in this case reads "120/70ZR17." This label indicates section width (120mm), aspect ratio (70 percent), speed rating (Z), construction (R), and wheel diameter (17 inches). " Motorcycle tire codes and markings. What do they mean? - Sport Rider

120 - section width (120mm)
70 - aspect ratio (70 percent)
ZR - speed rating and construction
17 - wheel diameter (17 inches)

TW200:

"The front tire was 130/80-18! I've had sportbikes with this as a REAR tire which could happily exceed 90MPH! (although sans the knobbies!). The rear tire was a whopping 180/80-14! " Flyin' Al's Yamaha TW200

front:
130 - 130mm width (5.1 inches)
80 - aspect ratio (80 percent)
104 - tire height (rim to tread) (130 * .80)
18 - 18 inches rim diameter

rear:
180 - 180mm width (7 inches)
80 - aspect ratio (80 percent)
144 - tire height (rim to tread) (130 * .80)
14 - 14 inches rim diameter
25.2 - inches tire diameter (2 * 144mm tire height + 14 inch rim diameter)

"the tire size is correctly represented as "235/30R22 90Y", not 235/30x22.

235 is the "section width" of the tire or its width measured from mid sidewall to mid sidewall in milimeters

30 is the aspect ratio or the measurement of the tire from the bead to the top of the tread expressed as a percentage of the section width - or 30% of 235mm.

R means the tire is of radial construction

22 means the tire fits a 22-inch diameter wheel. Add this number to the aspect ratio x2 and you arrive at the overall diameter of the tire.

90 is the Load Index and tells us if this tire can safety carry the load of your vehicle. Sicne we don't know what your vehicle is we don't know the answer to that question but I'm going to take a wild guess that this load index is much lower than what your car calls for.

Y is the speed rating and tells us how high a speed the tire can safely sustain under test conditions." (What do the numbers in tire sizes mean?like 235/30x22? - Yahoo! Answers)

"the only other thing to cover is why you would want / not want the high percent... a 30 is going to give you less rubber thickness... more wheel.. so you will have less cushion and feel the road more...a 65 or so will give you lots of cushion and less road feel" (What do the numbers in tire sizes mean?like 235/30x22? - Yahoo! Answers)

Car Bibles : The Wheel and Tyre Bible Page 1 of 4:

"How to read your tyre sidewall markings

This is probably the number one question I get asked - "how do I read my tyre?". It's confusing isn't it? All numbers, letters, symbols, mysterious codes. Actually, most of that information in a tyre marking is surplus to what you need to know. So here's the important stuff: "