Tyre data calculator
The TRC calculator can convert the tyre-geometrics from the tyre sizes to the ETRTO standard (European Tyre and Rim Technical Organisation).
NB. Your Browser must be able to run Java to be able to use the program. An explanation for each computed value can be found at the foot of the page after clicking 'Calculate'. To calculate the tyre-geometric data, we need the following input:
Tyre Width, the Aspect Ratio (SH/SW) and the Rim Diameter.

Input size
Value Tyre 1
Value Tyre 2
Comment, e.g. Make of Tyre, -type (optional)
Rim diameter in Inches (e.g. 17" as in 8Jx17)
SH/SW (Aspect) in % (e.g. 55% as in 205/55 R 15) **
Tyre width in mm (e.g. 205 mm as in 205/55 R 15)
Summer- (1), Winter tyres (2), Spare(VW, Audi "Solid") (3)

Output sizes Result Tyre 1 Result Tyre 2
Tyre rolling circumferance
Static rolling radius
Dynamic rolling radius
Speedo deviation at 100 km/h from Tyre 1 to ref. tyre (185/65 R15) and Tyre 2 to Tyre 1
Tyre suspension comfort value
Tyre volume

Change from mm into inch and vice versa
(please insert the known value into the appropriate box and click or tab to other box):
 mm   Inches 

Explanation of the input data

The Tyre width is in mm and is shown as the first set of numbers on the tyre wall, for example; 205 mm as on a tyre size 205/55 R 16 V.

The Aspect Ratio or tyre height/width ratio (The Aspect ratio) is measured as followed section hight of tyre (SH) divided by the section width of tyre (SW) and this gives the result as a percentage. A tyre with the size 205/50 ZR 16 has an Aspect Ratio of 50%. ** If no value is given, for instance as in 165 R 15S, then the Aspect Ratio is 80%.

The rim diameter is usually in inches. A rim size of 8Jx17 has a diameter of 17" or 431.8 mm.
Explanation ofthe output data
The following details for the tyre size are valid for those in ETRTO-Standard laid down for tyre load and tyre air-pressures,assuming no wear on standard profile.

The Tyre Rolling Circumference is the length of one rotation of the wheel (circumference), with the axle-weight influencing the result. Realistic values for car tyres are approx 2m.

The static rolling radius is the distance from the middle of the wheel to the road surface when the car is stood still. Realistic values for car tyres amount to roughly 30 cm.

The difference between the static rolling radius & the dynamic rolling radius is that the distance from the middle of the wheel to the tarmac is calculated for a speed of 60 km/h (36.7 mph). Higher speeds cause the tyre to slightly inflate and increase this value

For alternative tyres this is the speedo-deviation at 100 km/h compared to the original; for instance 185/65 R 15 is used as a reference for Tyre 1. The speedo-deviation cannot be more than +8% or +8km/h at 100 km/h (56mph) by law.

For the tyre suspension comfort value, this is more important for low cross-section tyres (low AR). These values are from 0 (very bad) up to 10 (very good). The value depends on the height of the tyre shoulder and this dictates the suspension comfort using the base values gained from trials with different tyres. These values are not the result of any ETRTO standards

The value for the tyre volume is the volume of air between tyre and rim.