Overall Diameter & Rolling Circumference
Sidewall height = Width × Aspect ratio%. Overall diameter = 2 × Sidewall + Rim × 25.4 mm. Rolling circumference = D × π. These values control speedometer display and actual speed.
Tire and Rim Calculator
Tires & Rims — will it fit?
Compares two tire and rim combinations for Swiss vehicles. Calculates outer diameter, rolling circumference, speedometer deviation according to UN-ECE R39, and track widening due to changed offset (ET). Provides clear guidance on when a rim expert opinion or an additional DTC expert opinion 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
Outer Track 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 tolerance rangeRolling circumference: +0.38%.
Rim/Track Tolerance
Rim — within tolerance rangeET 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 these, it follows how the wheel rolls, how the track is set, and whether the speedometer remains within legal limits.
UN-ECE R39
The speedometer continues to calculate with the original circumference. Only an indicated speed that is too high (max. 10% + 4 km/h) is permitted, never one that is too low — the basis for tire tolerance assessment.
ET, Expert Opinion & DTC
Enter the lowest ET permitted by the manufacturer as the original ET. A deviation of ±6 mm or more requires a rim expert opinion. For deviations greater than ±15 mm, a DTC expert opinion is additionally required.
Note and Disclaimer. This calculator is for general information purposes only. The calculations are based on UN-ECE R39 and common ETRTO tolerances, but do not claim to be complete, current, or legally binding. They do not replace a rim or DTC expert opinion, nor an inspection by the Road Traffic Office. No liability is assumed for decisions made based on the values displayed here.