Sizing tools
Know the exact size before you buy
Tools that compute, compare and recommend sizes — so you get it right the first time.
Width
Aspect Ratio
Wheel
R
The tools
Three focused tools that answer every sizing question.
Size calculator
Decode any size into its full spec in mm, with the concentric visualiser and a built-in size guide.
Answers
"What does 205/55R16 actually mean?"
Diameter & circumference
Revs/km
Size guide
Compare two sizes
Overlay two sizes to true scale and see diameter, circumference and speedometer error side by side.
Answers
"What changes if I switch to 225/45R17?"
Visual overlay
Speedometer error
Plus-sizing & alternates
Every alternative and plus-size per rim, ranked by fitment safety with the speedometer error.
Answers
"Will this fit my rim, and is it safe?"
Will it fit?
Speedometer error
The safety rule
Stay inside the ±3% diameter window
Any alternative size should keep its overall diameter within 3% of the original — so your speedometer, odometer, gearbox and safety systems stay accurate.
≤ 1.5%
Safe
fit with confidence
≤ 3%
Acceptable
double-check first
> 3%
Out of range
not recommended