Quick Answer
- Pick your BMW, enter the size you want and the calculator compares it with the BMW factory size. For example the 3 Series (G20) base size 225/50R17 against 225/45R18 is +0.11 in (+0.4%), so an indicated 60 mph is 60.3 mph (calculated).
- Continental says to keep rolling circumference within 3% of the original. The model list uses sizes from BMW Group PressClub specifications.
- For xDrive, BMW publishes no tolerance; BMW Australia asks for as similar a rolling circumference as possible. BMW's own front and rear pairs differ by 0.07% to 0.72%.
① OEM Size
→ 225/50R17
② Your Size
→ 225/45R18
Select a BMW model above, enter your desired size, then click Calculate
Supports P-metric format (e.g. 225/45R18). xDrive models are checked against the 1% rolling diameter guideline.
BMW Factory Sizes and Example Swaps
Factory sizes are from BMW Group PressClub specifications. The swap sizes are examples, not BMW approvals, and the differences are calculated.
| Model | BMW size | Example swap | Diameter in | Difference | True speed at 60 mph |
|---|---|---|---|---|---|
| 3 Series (G20) base | 225/50R17 | 225/45R18 | 25.86 to 25.97 | +0.4% | 60.3 mph |
| M340i xDrive (G20) front | 225/45R18 | 225/40R19 | 25.97 to 26.09 | +0.4% | 60.3 mph |
| M3 (2025) 18 inch front | 275/40R18 | 275/35R19 | 26.66 to 26.58 | -0.3% | 59.8 mph |
| M3 / M4 (2025) 19 inch front | 275/35R19 | 285/35R19 | 26.58 to 26.85 | +1.0% | 60.6 mph |
| X3 (G45) xDrive | 245/50R19 | 245/45R20 | 28.65 to 28.68 | +0.1% | 60.1 mph |
| X5 (G05) xDrive 19 inch | 265/50R19 | 265/45R20 | 29.43 to 29.39 | -0.1% | 59.9 mph |
| X5 (G05) xDrive 20 inch | 275/45R20 | 285/45R20 | 29.74 to 30.10 | +1.2% | 60.7 mph |
For front against rear on xDrive use the BMW xDrive tire calculator; the sizes by model are in the BMW tire size guide.
How the Comparison Works, and Sources
Overall diameter
Rim + 2 x width x aspect / 100 / 25.4. For 225/45R18: 18 + 2 x 101.25 / 25.4 = 25.97 in.
Difference from the BMW size
Your diameter - BMW diameter, divided by the BMW diameter for percent. This is also the speedometer error.
Speedometer
True speed = indicated speed x your diameter / BMW diameter.
Front and rear pairs
On staggered models the calculator compares each axle against its own BMW size; the BMW xDrive calculator compares front with rear.
| Source | Statement (paraphrased) |
|---|---|
| BMW Group PressClub, technical specifications | Source of the factory tire sizes in the model list and the table below, several of them staggered on M and xDrive models. |
| BMW Australia, tyre FAQ | On xDrive models all four tyres should have as similar a rolling circumference as possible. No percentage is given. |
| Continental Tire, plus-sizing guidance | Keep rolling circumference within 3% of the original and use only approved size and rim width combinations. |
| NHTSA | If front and rear tires are different sizes you may not be able to rotate them, so check the owner's manual. |
How to Use the BMW Tire Size Calculator
- 1Select your BMW model: the factory front size fills in and the drive type is set.
- 2Enter the tire size you want in the Your Size panel (width, aspect ratio, rim).
- 3Click Calculate and Compare to see the diameter difference, speedometer error and revs per mile.
- 4For an xDrive car compare front with rear in the BMW xDrive tire calculator, and confirm any non-factory size with a BMW dealer.