Compare two tire sizes: outer diameter, difference in mm and %, ride-height change, actual road speed, and speedometer error.
Inputs
100 – 400
Section width of the original tire in millimetres — the first number in a tire spec like 225/45R18.
25 – 85
Sidewall height as a percentage of section width — the second number in 225/45R18 (45 means the sidewall is 45% of 225 mm).
10 – 24
Wheel rim diameter in inches — the number after the R in 225/45R18. Tire spec keeps this in inches even when the rest of the tire is metric.
100 – 400
Section width of the replacement tire in mm.
25 – 85
Sidewall percentage of the replacement tire.
10 – 24
Rim diameter of the replacement tire in inches.
0 – 300 km/h
The reading on your speedometer that you want to translate into actual road speed with the replacement tire fitted. Defaults to 60 km/h, a common reference speed.
Results
Diameter change as a percentage of the original. Most tire shops consider ±3% the safe limit before speedometer error, ABS calibration, and clearance become concerns.
Spec diameter of the original tire — rim in inches converted to mm plus twice the sidewall height.
Spec diameter of the replacement tire, calculated the same way as the original.
Replacement minus original outer diameter (mm). Positive means the new tire is taller.
Half the diameter difference — the change in fender-to-ground clearance with the new tire.
True ground speed when the speedometer reads the indicated value. Computed by scaling indicated speed by the diameter ratio (D2 / D1).
How far off the speedometer will be after the swap, in percent. A negative value means the speedometer over-reads (you are actually going slower than it shows).
What Is Tire Size Comparison?
Swapping tires is one of the most common modifications drivers make — for aesthetics, performance, or simply replacing a discontinued size. The critical question is how much the outer diameter changes, because that single number determines your speedometer accuracy, ABS calibration, and whether the tire clears your fenders.
How to Read a Tire Size
A tire spec like 225/45R18 encodes three measurements:
225 — section width in millimetres
45 — aspect ratio: the sidewall height is 45% of 225 mm = 101.25 mm
R18 — radial construction, rim diameter 18 inches
The outer diameter formula is:
D=Rin×25.4+2×W×100AD225/45R18=18×25.4+2×225×0.45=457.2+202.5=659.7 mm
Speedometer Error and Ride Height
When you fit a taller tire, each wheel rotation covers more ground. The speedometer — calibrated for the original tire circumference — reads low.
The ride-height change is simply half the diameter difference (one tyre radius on each side):
Δh=2D2−D1
Quick Reference: Common Plus-Sizing Pairs
Original
Replacement
Δ diameter
Error
205/55R16
215/50R17
+14.9 mm
+2.36%
225/45R18
245/40R19
+18.9 mm
+2.86%
235/55R17
255/50R18
+21.9 mm
+3.17%
All examples are within or near the ±3% safe tolerance.
Frequently Asked Questions (FAQ)
How do I read a tire size like 225/45R18?
225 is the section width in mm. 45 is the aspect ratio — the sidewall is 45% of 225 mm = 101 mm tall. R means radial construction. 18 is the rim diameter in inches. Outer diameter = 18 × 25.4 + 2 × 101 ≈ 659 mm.
Why does a different tire size change my speedometer?
A speedometer measures wheel rotations and multiplies by the calibrated tire circumference. A taller tire covers more ground per rotation, so the car actually goes faster than the speedometer says. A shorter tire is the reverse.
How much diameter change is safe?
±3% is the conventional tolerance — it keeps speedometer error inside legal limits in most jurisdictions and avoids ABS/traction-control miscalibration. Beyond ±3%, also check fender clearance, suspension travel, and load index.
What is "plus sizing"?
Plus-one (or plus-two) sizing fits a larger-rim, lower-profile tire while keeping outer diameter nearly identical. The goal is sharper steering response and a more aggressive look without affecting speedometer, gearing, or clearance.
Disclaimer
Calculator assumes the manufacturer-spec outer diameter; real-world loaded diameter is 2–4% smaller due to sidewall flex. Always cross-check with manufacturer load and clearance data before fitting.
