Overall Diameter & Rolling Circumference
Sidewall height = Width × Aspect ratio%. Overall diameter = 2 × Sidewall + Rim × 25.4 mm. Rolling circumference = D × π. These values control the speedometer reading and actual speed.
Tire & wheel calculator
Tires & Wheels — do they fit?
Compares two tire and wheel combinations for Swiss vehicles. Calculates outer diameter, rolling circumference, speedometer deviation according to UN-ECE R39, and track widening due to modified offset (ET). With clear indication of when a wheel certificate or an additional DTC certificate is required.
UN-ECE R39 Speedometer limits
ETRTO Tolerances
0% Advertising & Tracking
Original Combination
Tire
mm
%
″
Rim
J (″)
mm
New Combination
Tire
mm
%
″
Rim
J (″)
mm
Tire values (detail)
Outer Diameter
634.3 mm
+2.4 mm · +0.38%
Rolling Circumference
1992.7 mm
+7.5 mm · +0.38%
Sidewall Height
101.3 mm
−11.5 mm · −10.20%
Tire Width
225 mm
+20 mm · +9.76%
Rim & Track Values (Detail)
ET Difference
−5 mm
Wheel moves 5 mm outwards
Rim Width Δ
+25.4 mm
+1.0 J
Track outer per side
+17.7 mm
Towards fender
Total track per axle
+35.4 mm
Track widening
Visual Comparison · Top-Down View
Original
New
Tire Tolerance
Tire — within toleranceRolling circumference: +0.38%.
Rim/Track Tolerance
Rim — within toleranceET change: −5 mm.
Speedometer Deviation
At speedometer reading
100 km/h
100.4km/h actual
+0.4 km/h
How it's calculated — and why it matters.
Three numbers for the tire, two for the rim. From this, it follows how the wheel rolls, how the track sits, and whether the speedometer remains within legal limits.
UN-ECE R39
The speedometer continues to calculate with the original circumference. Only a reading that is too high is permitted (max. 10% + 4 km/h), never one that is too low — the basis for tire tolerance assessment.
ET, Expert Reports & DTC
Enter the lowest ET permitted by the manufacturer as the original ET. A deviation of ±6 mm or more requires a rim expert report. More than ±15 mm additionally requires a DTC expert report.
Note and Disclaimer. This calculator is for general information only. The calculations are based on UN-ECE R39 and usual ETRTO tolerances, but do not claim to be complete, up-to-date or legally binding. They do not replace a rim or DTC expert report or an inspection by the Road Traffic Office. No liability is assumed for decisions made on the basis of the values displayed here.
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