Question 1

How do I calculate the tire diameter from the tire size?

Accepted Answer

Multiply the width by the aspect ratio as a percentage to get the sidewall height. For 205/55 R16 that is 205 × 0.55 = 112.75 mm. The diameter is the rim in millimetres plus twice the sidewall: 16 × 25.4 + 2 × 112.75 = 631.9 mm. The circumference is diameter × π, here about 1,985 mm.

Question 2

How much can the rolling circumference differ with another tire size?

Accepted Answer

There is no fixed legal percentage. The tire trade's rule of thumb is: at most about 1.5% larger and 2% smaller than the original size. What is binding, however, is only what the EC certificate of conformity (CoC), a type approval or an expert report states for your car. Other sizes must be checked by an expert.

Question 3

Why does the speedometer show less with larger tires?

Accepted Answer

The speedometer counts wheel revolutions. A larger tire covers more distance per revolution, so at the same reading you are going faster. With +2% circumference the speedometer shows 100 km/h while you are really doing 102 km/h. Under UN ECE R39 a speedometer must never show less than the true speed.

Question 4

Do I need to have a new tire size registered?

Accepted Answer

Not if the size is listed in your vehicle papers. Many sizes are not in the registration certificate but in the CoC document. If the size is missing there, you need a type approval (ABE) or a parts report. With a parts report an inspection by an expert is usually required. Without it the operating permit can lapse (§ 19 StVZO).

Question 5

What does the offset (ET) of a wheel mean?

Accepted Answer

The offset is the distance between the wheel's centre line and the mounting face at the hub, in millimetres. A lower offset pushes the wheel outwards: the track gets wider, by twice the difference per axle. Going from ET 45 to ET 35 widens the track by 20 mm. A wider rim also reaches half the extra width further in and further out.

Question 6

How do I find out how old my tires are?

Accepted Answer

Look for "DOT" on the sidewall. The last four digits in an oval show the production week and year. 2219 means calendar week 22 of 2019. Tires with only three digits were made before 2000. Enter the digits in the calculator above to see the exact age.

Question 7

What is the maximum age for car tires?

Accepted Answer

There is no legal maximum age for car tires in Germany. Rubber hardens over the years, even if the tread is still deep enough. Experts recommend having tires checked every year from about six years and replacing them after ten years at the latest.

Question 8

What do the load index and speed rating mean?

Accepted Answer

After the size you will see something like "91V". 91 is the load index: 615 kg per tire. V is the speed rating: up to 240 km/h. New tires must meet at least the values in your vehicle papers. Only winter tires may have a lower speed rating, provided a notice is displayed in the driver's field of view.

Question 9

Is there a winter tire requirement in Germany?

Accepted Answer

Yes, but depending on conditions: in black ice, packed snow, slush, ice or frost you may only drive on winter or all-season tires (§ 2(3a) StVO). Since 1 October 2024 only tires with the Alpine symbol (mountain with snowflake) count. Driving without them costs €60 and one penalty point.

Guide: Tire Size & Wheel Calculator

Tire Size Explained: Diameter, Circumference and Alternative Sizes

How to Read Your Tire Size: What the Numbers and Letters Mean

Speedometer Error from Different Tires: Working Out Your Real Speed

Tire Age from the DOT Code: How Old Can Tires Be?

Winter Tire Rules in Germany: Alpine Symbol, Fines and Tread Depth

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