How motorcycle tire sizes are written
Most modern motorcycle tires use the same metric code as cars: 180/55ZR17 is 180 mm wide, with a sidewall 55% of the width, on a 17 in rim. ZR and R mean radial construction, and a dash, as in 130/90-16, means bias ply. The load index and speed rating that follow (for example 73W) do not change the dimensions. This calculator reads all three styles. Older letter codes like MT90-16 and inch codes like 4.00-18 use a different system and are not supported here.
The formula
sidewall (mm) = width × aspect ÷ 100
diameter (in) = rim + 2 × sidewall ÷ 25.4
circumference = π × diameterWorked example: 180/55ZR17 to 190/50ZR17
The stock rear has a sidewall of 180 × 0.55 = 99 mm, so its diameter is 17 + 2 × 99 ÷ 25.4 = 24.80 in (630 mm) and its circumference 77.9 in. The 190/50ZR17 has a 95 mm sidewall: 24.48 in (622 mm) tall, 76.9 in around. It is 0.39 in (10 mm) wider but 1.3% shorter, and the rear axle sits about 0.16 in lower. If the bike reads speed from the rear wheel, 60 mph on the dial becomes 59.2 mph real.
A 190/55ZR17 goes the other way: 25.23 in tall, 1.7% taller than the 180/55, which makes a rear-wheel speedometer read low (60 on the dial is 61.0 mph).
Why the speedometer effect depends on the bike
A speedometer only sees the wheel or shaft its sensor reads. If your bike reads speed from the front wheel, a new rear size does not change the reading at all, though it still changes the overall gearing a little. If it reads from the rear wheel or the gearbox output, the rear tire change moves the reading by the same percentage as the diameter. The table above shows both cases.
Ride height and handling
A change in diameter raises or lowers that end of the bike by half the difference. A lower rear drops the tail, which tends to slow the steering slightly; a taller rear tends to do the opposite. The numbers are small, but on a sport bike riders notice them. A wider rear tire also needs a suitably wide rim and room at the swingarm and chain.
Questions
Can I put a 190/50ZR17 instead of a 180/55ZR17?
By the numbers, the 190/50 is 0.39 in wider and 1.3% shorter (24.48 in vs 24.80 in). Whether it suits your rim and clears the swingarm and chain is for the tire maker's fitment data and your service manual.
What is the diameter of a 120/70ZR17 motorcycle tire?
The sidewall is 120 × 0.70 = 84 mm, so the diameter is 17 + 2 × 84 ÷ 25.4 = 23.61 in (600 mm) and the circumference 74.2 in.
Does a bigger rear tire change my speedometer?
Only if your bike reads speed from the rear wheel or the gearbox. Going from 180/55ZR17 to 190/55ZR17 makes such a speedometer read 1.7% low. If it reads from the front wheel, the rear size does not affect it.
What does the dash in 130/90-16 mean?
Bias ply construction. An R or ZR in the same place means radial. The dimensions are calculated the same way: 130/90-16 is 25.21 in tall.