buncha/engineering/tyre-size

Tyre Size Calculator

Compare the tyres you have against a size you're thinking about. See how the overall diameter, sidewall, ride height and revs change — and how far your speedometer would be out, since it's calibrated to the original size.
205/55R16 to 225/45R17
+0.4%
diameter vs your current tyre (+2.4 mm)
New diameter
634 mm
24.97 in
Ride height
+1.2 mm
at the axle
Speedo at 60 mph
60.2
mph actual
Within spec
Under the ±3% rule most fitment guides use — speedo and gearing barely change.

Current tyre

205/55R16
Width
mm
Profile
%
Rim
in
Popular sizes

New tyre

225/45R17
Width
mm
Profile
%
Rim
in

Side by side, to scale

Current — 205/55R16New — 225/45R17
Both tyres drawn sitting on the same road. The gap at the top is how much taller (or shorter) the new tyre stands.

The numbers

CurrentNewChange
Overall diameter631.9 mm634.3 mm+2.4 mm
Diameter (inches)24.88 in24.97 in+0.09 in
Sidewall height112.8 mm101.3 mm-11.5 mm
Section width205 mm225 mm+20 mm
Circumference1.985 m1.993 m+0.8 cm
Revolutions / mile811808-3
True speed at 60 mph indicated60 mph60.2 mph+0.2 mph
A planning estimate from the size numbers alone — real rolling diameter varies a little by tyre model, load, pressure and wear, and actual fit depends on your wheel offset, guard clearance and any speed-rating or load-index requirements. The ±3% guideline is a rule of thumb, not a legal limit; check your owner's manual and local rules. Everything runs in your browser.

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About Tyre Size Calculator

Compare two tyre sizes and see exactly what changes. Enter your current size and a new one (width / profile / rim) to get the overall diameter, sidewall height, circumference, revs per km or mile, ride-height change and the speedometer error the swap introduces — with a to-scale side-by-side diagram and a ±3% fit verdict. Works for plus-sizing, going taller for clearance, or just checking a set you found cheap.

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