Tire Size Comparison Calculator
Compare two tire sizes by diameter, sidewall, circumference and revs per mile. See speedometer changes in mm, inches and percent.
Compare two tire sizes.
Tire Size Comparison Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Tire Size Comparison Calculator Works?
The Tire Size Comparison Calculator works by converting two size codes into millimeter dimensions and subtracting the stock values from the new values. The comparison runs in 4 steps.
Enter the stock size
Type the width, aspect ratio and rim diameter printed on the current sidewall, for example 225/45 R17, as Tire 1.
Enter the proposed size
Type the size under consideration, for example 225/40 R18, as Tire 2.
Read the 6 dimension rows
The calculator lists section width, sidewall height, overall diameter, circumference, revolutions per mile and ride height change for both tires.
Check the percentage gap
The calculator flags the diameter difference in green inside ±3% and in amber outside ±3% of the stock tire.
Tire Size Comparison Calculator Formula
The Tire Size Comparison Calculator formula is Difference % = (OD₂ − OD₁) ÷ OD₁ × 100, where each overall diameter (OD) comes from the ISO metric size code.
SH = W × (AR ÷ 100)OD = 2 × SH + (Rim × 25.4)ΔOD = OD₂ − OD₁Δ% = ΔOD ÷ OD₁ × 100- W
- Section width in millimeters (mm)
- AR
- Aspect ratio in percent
- Rim
- Rim diameter in inches (in)
- OD₁, OD₂
- Overall diameter of stock and new tire in mm
Worked example
A 225/45 R17 tire measures 2 × 101.25 + 431.8 = 634.3 mm (24.97 in). A 225/40 R18 tire measures 2 × 90 + 457.2 = 637.2 mm (25.09 in). The difference is +2.9 mm (+0.11 in), which equals +0.46%.
Tire Size Comparison Calculator Conversion Table
The Tire Size Comparison Calculator conversion table shows 8 sizes measured against a 225/45 R17 baseline of 634.3 mm (24.97 in).
| New size | Overall diameter | Difference vs 225/45 R17 | True speed at indicated 60 mph (96.6 km/h) |
|---|---|---|---|
| 225/45 R17 | 634.3 mm (24.97 in) | +0.00% | 60.0 mph (96.6 km/h) |
| 225/40 R18 | 637.2 mm (25.09 in) | +0.46% | 60.3 mph (97.0 km/h) |
| 235/40 R18 | 645.2 mm (25.40 in) | +1.72% | 61.0 mph (98.2 km/h) |
| 245/40 R18 | 653.2 mm (25.72 in) | +2.98% | 61.8 mph (99.4 km/h) |
| 225/35 R19 | 640.1 mm (25.20 in) | +0.91% | 60.5 mph (97.4 km/h) |
| 245/35 R19 | 654.1 mm (25.75 in) | +3.12% | 61.9 mph (99.6 km/h) |
| 215/50 R17 | 646.8 mm (25.46 in) | +1.97% | 61.2 mph (98.5 km/h) |
| 235/45 R17 | 643.3 mm (25.33 in) | +1.42% | 60.9 mph (97.9 km/h) |
What is Tire Size Comparison Calculator?
A Tire Size Comparison Calculator is a free online tool that places two tire sizes next to each other and reports the dimensional and speedometer differences between them. The tool reads the standard metric code (width/aspect ratio R rim) defined by the International Organization for Standardization (ISO) and the Tire and Rim Association (TRA).
The calculator returns nominal values. Nominal values are the dimensions implied by the size code on an unloaded, correctly inflated tire. Measured tires vary by 1% to 2% between brands, and a tire maker's published specification sheet gives the exact figure for one model.
Drivers, tire shops and wheel retailers use the Tire Size Comparison Calculator before buying tires, before fitting aftermarket wheels and before converting to winter or off-road setups.
What are the uses of Tire Size Comparison Calculator?
There are 6 main uses of the Tire Size Comparison Calculator, including plus-size checks, speedometer checks and clearance checks.
Plus-size planning
Confirm that a larger wheel with a lower-profile tire keeps the overall diameter within ±3% of stock.
Speedometer error checks
Read the true road speed at an indicated 60 mph (96.6 km/h) before changing tire size.
Ride height estimates
See how far the axle centerline rises or drops, which equals half of the diameter change.
Replacement shopping
Compare an out-of-stock factory size with sizes the tire store has on hand.
Off-road upsizing
Measure how much taller an all-terrain tire stands before checking fender clearance.
Winter setups
Compare a narrower winter size against the summer size on the same vehicle.
Related Core Tire Dimensions Tools
View All 134 Subtools →Compare several sizes together.
Estimate nominal overall diameter.
Calculate absolute and percentage differences.
Estimate nominal circumference.
Compare estimated rolling distances.
Compare nominal section widths.