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

	Current	New	Change
Overall diameter	631.9 mm	634.3 mm	+2.4 mm
Diameter (inches)	24.88 in	24.97 in	+0.09 in
Sidewall height	112.8 mm	101.3 mm	-11.5 mm
Section width	205 mm	225 mm	+20 mm
Circumference	1.985 m	1.993 m	+0.8 cm
Revolutions / mile	811	808	-3
True speed at 60 mph indicated	60 mph	60.2 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.

Everything happens on your device. Close the tab and it's gone.