Tire Size Calculator
Convert a tire designation (e.g. 205/55R16) into real-world geometry and compare two sizes to see the speedometer error if you switch.
Diameter difference
7.04%
- Original tire diameter
- 24.88
- New tire diameter
- 26.63
- Original sidewall (mm)
- 112.75
- New sidewall (mm)
- 135
- Original circumference (in)
- 78.16
- New circumference (in)
- 83.66
- Original revs per mile
- 811
- New revs per mile
- 757
- True speed at indicated 60 mph
- 64.23
Each tire spec WIDTH/ASPECT R RIM is converted to overall diameter: sidewall height = width × aspect ÷ 100 (mm), then diameter = rim × 25.4 + 2 × sidewall. A positive diameter difference means the new tire is taller, so it covers more ground per revolution — the speedometer (calibrated to the original tire) under-reports speed by the same percentage. True speed at an indicated 60 mph equals 60 × new diameter ÷ original diameter.
How to use this calculator
Enter the three numbers from each tire sidewall: section width in millimetres, aspect ratio (the percent after the slash), and wheel diameter in inches (the number after the R). The left column is your current/OEM tire, the right column is the size you are considering. The calculator shows the diameter difference as a percentage, the speedometer reading at an indicated 60 mph, and the full geometry breakdown for each tire.
How the calculation works
A tire stamped 205/55R16 has a section width of 205 mm, a sidewall height equal to 55% of the width (112.75 mm), and fits a 16-inch wheel. Overall diameter equals the wheel diameter (converted to mm with 1 in = 25.4 mm) plus two sidewall heights. Circumference is π × diameter. Revs per mile is 63,360 in (the number of inches in a mile) divided by circumference in inches. Vehicle speedometers measure axle rotations and multiply by the original tire circumference — fit a larger tire and you cover more ground per turn, so the speedometer reads low. True speed equals indicated speed × (new diameter ÷ original diameter).
Worked example
Replacing a 205/55R16 with a 225/60R16. Original sidewall = 205 × 0.55 = 112.75 mm, diameter = 16 × 25.4 + 2 × 112.75 = 631.9 mm = 24.88 in. New sidewall = 225 × 0.60 = 135 mm, diameter = 16 × 25.4 + 270 = 676.4 mm = 26.63 in. Diameter difference = (26.63 − 24.88) ÷ 24.88 = +7.04%. At an indicated 60 mph the speedometer is reading the original calibration, but the wheels are turning slower for any given speed, so true speed = 60 × 26.63 ÷ 24.88 = 64.2 mph. Revs per mile drop from 810 to 757 — about 53 fewer rotations per mile.
Frequently asked questions
How do I read a tire size like 205/55R16?
The first number (205) is the section width in millimetres. The second number (55) is the aspect ratio — the sidewall height as a percentage of the section width. The R means radial construction (almost every passenger tire today). The final number (16) is the wheel diameter in inches. Some sizes add a load index and speed rating after, e.g. 205/55R16 91V, but those do not affect the geometry the calculator returns.
How much speedometer error is acceptable?
Most regulators allow speedometers to read up to about 10% high but never low. UN ECE Regulation 39 (UK / EU / much of the world) caps it at indicated ≤ 1.1 × true + 4 km/h. US FMVSS 101 has no hard tolerance but a common OEM target is ±2.5%. As a working rule, keep new-vs-original tire diameter within 3% and you stay inside both. Beyond about 5% you should expect noticeable speedometer drift and possible ABS / traction control miscalibration.
Does a wider tire have a bigger diameter?
Only if the aspect ratio stays the same. Sidewall height = width × aspect ÷ 100, so a 225/55R16 is taller than a 205/55R16 (sidewall goes from 112.75 mm to 123.75 mm). But a 225/45R16 is shorter than a 205/55R16 because the aspect ratio dropped from 55 to 45, more than offsetting the wider tread. Always recompute the diameter — width alone does not tell you whether the tire grew or shrank.
Why does changing tire size affect revs per mile?
Revs per mile is just 63,360 inches divided by the tire circumference, so it scales inversely with diameter. A 5% larger diameter means about 5% fewer revolutions per mile. Cruise control, ABS, traction control, odometer, and fuel economy readouts all derive from those wheel rotations. The car still drives, but the numbers on the dash drift away from reality until the speed sensor is recalibrated or the ECU is reflashed for the new tire.
Can I use this calculator for metric or imperial sizes?
It uses the global P-metric (P-Metric) tire designation — width in millimetres, aspect ratio as a percentage, wheel in inches — which is by far the most common system worldwide. For pure imperial Light Truck flotation sizes like 33×12.50R15 you would convert: enter the diameter and section width as if you were specifying the equivalent metric size. Pure-metric Euro sizes like 195R14C work too — they imply an 80-series aspect ratio if none is stamped.
Will a 3% taller tire ruin my gearing?
It will lengthen every gear by 3% — first gear feels slightly weaker, top gear cruises slightly lower revs. Most cars tolerate this without issue and many drivers actively choose taller tires for highway fuel economy. The bigger concerns are clearance (does the tire rub at full steering lock or full suspension compression?), load rating (does the new size meet the placard load index?), and ABS / TC behavior — modern stability systems assume the OEM rolling diameter.