Automotive Reference
Every number on your sidewall, decoded — then a calculator that tells you the real diameter and whether a new size will throw off your speedometer.
Rule of thumb: keep overall diameter within about 3% of stock, or your speedometer, odometer and (on many cars) the traction/ABS systems read wrong. Taller tires make the speedo under-read your true speed.
The last two characters — 95H in the example — are the service description: how much weight the tire carries and how fast it's rated to run.
| Load index | Max load (per tire) |
|---|
Never fit a lower load index than the placard in your door jamb.
| Speed rating | Max sustained |
|---|
Match or exceed the original rating, especially on AWD/performance cars.
215 is the tread width in millimetres, 65 is the aspect ratio (the sidewall is 65% of the width, so ~140 mm tall), R means radial construction, and 16 is the wheel diameter in inches. That size works out to about a 27-inch overall diameter — use the calculator above for the exact figure.
If the overall diameter changes, yes. A taller tire makes the speedometer read slower than you're actually going (and vice-versa). Under ~3% difference is generally fine; beyond that, speed, odometer and mileage all drift. The calculator shows the exact error.
Often, within a range — a 16-inch wheel typically takes a spread of widths. But wider tires can rub or change the diameter, so check clearance and keep the overall diameter close to stock. Widening usually means dropping the aspect ratio to hold the same height (e.g. 215/65 → 235/60).
Speed rating — the max sustained speed the tire is built for. T is 118 mph, H is 130, V is 149, W is 168. It's not a suggestion to drive that fast; it reflects how the tire is constructed. Match or exceed your car's original rating.