🛞 Tire Size Calculator
Turn a commercial or light-truck tire size into real dimensions — diameter, width, sidewall, circumference, and revs per mile — and compare two sizes to see the speedometer and gearing effect before you fit them.
🛞 Current / calibrated size
🛞 New / comparison size
↔ Speedometer & gearing effect
A taller tire under-reads the speedometer and effectively raises your gearing. General estimates — confirm load and speed ratings and clearances against your OEM specs and a tire professional before fitting a different size.
Size changes ripple through the whole truck
Rolling diameter sets your effective gearing, your speedometer and odometer accuracy, and how the ABS and driveline read wheel speed. Even a size that looks close can shift revolutions per mile enough to matter across a fleet's fuel and maintenance records.
Use the comparison to keep a swap within a sensible diameter tolerance, then confirm load range, speed rating, and clearance for your specific axle and body.
❓ Frequently Asked Questions
How do I read a tire size like 295/80R22.5?
295 is the section width in millimetres, 80 is the aspect ratio (sidewall height as a percentage of width), R means radial, and 22.5 is the rim diameter in inches. So this tire is 295 mm wide with a sidewall of 295 × 0.80 = 236 mm, mounted on a 22.5-inch commercial wheel.
How is overall diameter worked out?
Overall diameter = rim diameter (converted to millimetres) + two sidewalls. For 295/80R22.5 that is 22.5 × 25.4 + 2 × 236 = 1043.5 mm (about 41.1 inches). Circumference is π × diameter, and revolutions per mile is 63,360 ÷ the diameter in inches ÷ π.
Why does changing tire size affect my speedometer?
The speedometer counts wheel revolutions and assumes a fixed rolling diameter. A taller tire covers more ground per turn, so you travel faster than the needle shows (it under-reads); a shorter tire does the opposite. The comparison shows the percentage difference and your corrected speed.
Can I rely on this for fitment?
These are general estimates of geometry only. Load range, speed rating, and physical clearance still have to match your axle and body. Verify against your OEM specs and confirm any size change with a qualified tire professional; this is not professional fitment advice.