Compare two tire sizes — see diameter change, sidewall difference, and speedometer correction.
Enter two tire sizes and press Compare.
Tire sizes follow the format Width/Aspect R Rim — for example, 225/45R17 means
225 mm wide, with a sidewall that is 45% of the width, on a 17-inch rim. Total diameter is
calculated as: (rim × 25.4) + (width × aspect/100 × 2).
When switching tire sizes on a BMW E36, the most important number is overall diameter. A larger diameter raises the actual speed versus what the speedometer reads; a smaller one does the opposite. The BMW factory tolerance is ±1.5%, but most TÜV guidelines allow up to 3%. Going beyond that risks failed inspection and inaccurate ABS calibration.
On the E36, the stock 16-inch setup is 205/55R16 (634 mm diameter). Moving to a 225/45R17 gives 632 mm — virtually identical, which is why it remains the most common upgrade. Stretching to a 215/35R18 drops diameter to around 610 mm, which is a 3.8% change and will push the speedometer high by about 4 km/h at every indicated speed.