Drivetrain
Compare two tire sizes side-by-side: overall diameter, circumference, revs per mile, speedometer error percentage, and effective gear ratio change. Indispensable before a tire upsize.
A tire size calculator compares metric tire specs (like 275/70R17) to overall diameter, speedometer error, and effective gear change when you upsize wheels on a truck or plus-size a sports car. Taller tires lower RPM and make the speedo read slow; shorter tires do the opposite.
Run it before you buy lift kits, regear, or calibrate a PCM. Jeep and half-ton owners use it constantly when moving from stock metric sizes to 33″ or 35″ equivalents.
A good outcome: percent speedometer error and effective axle factor to feed the speedometer correction and gear ratio calculators.
Enter stock size first, new size second — error shows how the speedometer will read with the new tire.
Published metric sizes are nominal; two brands of “285/70R17” can differ slightly in true height — measure if you are near clearance limits.
Flotation sizes (33×12.50R17) are approximate — enter closest metric or measured diameter.
Plus-sizing wheels changes sidewall height even when overall diameter is “close” — always compare calculated diameters.
Metric tire overall diameter (inches):
Diameter = Rim + 2 × (Width × Aspect% ÷ 100) ÷ 25.4
Example: 275/40R18 → sidewall = 275 × 0.40 = 110 mm → 110×2/25.4 ≈ 8.66″ + 18″ rim ≈ 26.7″ tall.
Speedo error % = (New diameter − Old diameter) ÷ Old × 100
Effective gear factor = Old diameter ÷ New diameter (values > 1 = taller tire, lower RPM).
Overall diameter (in): Rim + 2 × (Width × Aspect% ÷ 100) ÷ 25.4.
Example: 275/40R18 → sidewall 110 mm × 2 ÷ 25.4 + 18″ rim ≈ 26.7″.
Speedo error % = (New − Old) ÷ Old × 100. Effective gear factor = Old diameter ÷ New diameter (values > 1 = taller tire, lower RPM).
Stock many F-150s wear 265/70R17; a common upgrade is 285/70R17. The calculator shows a taller overall diameter, a few percent speedometer error, and a slight effective gearing change — the usual trio shop owners check before re-gearing.
Moving from 235/50R17 to 275/40R18 on an S197/S550 Mustang changes diameter and revs per mile. Speedo correction and effective axle ratio help explain why the car “feels taller” on the highway after plus-sizing wheels.
A 285/70R17 metric tire is a close stand-in for a labeled 33×12.50R17 flotation size. Compare against stock 255/75R17 to see why speedometer and gear correction modules matter after a lift.
Published tire dimensions are nominal — a 275/70R17 often measures shorter or taller than calculated when mounted and loaded. Wear, brand, and load rating change effective diameter. Always measure rolling circumference or overall height on the vehicle.
Plus-sizing wheels changes revs per mile even when “overall diameter is close.” Compare stock vs new in this tool, then confirm with GPS vs speedometer and adjust PCM or speedo gear if needed.
A few percent is common — e.g., moving from 265/70R17 to 285/70R17 often reads roughly 2–4% slow (true speed higher than indicated) depending on actual tire height.
Yes. Taller tires act like a taller (lower numeric) axle ratio — less acceleration, lower RPM at cruise. The effective gear factor in this calculator quantifies that.
Flotation labels are approximate. Enter the closest metric equivalent or measured diameter in inches for accurate speedo and gearing math.
Pair this calculator with these related tools in the garage — same session, no signup.