Reading a Tire Size
A sidewall marking such as 225/50R17 encodes three numbers:
| Part | Means | In this example |
|---|---|---|
| 225 | Section width in millimetres | 225 mm wide |
| 50 | Aspect ratio: sidewall height as a percentage of width | Sidewall is 112.5 mm |
| R | Construction: R for radial | Radial |
| 17 | Wheel diameter in inches | Fits a 17″ wheel |
The aspect ratio is a ratio, not a fixed height. This is why widening a tire without lowering the aspect ratio makes it taller as well: 245/50R17 has a sidewall of 122.5 mm rather than 112.5 mm, adding almost 0.8″ to the overall diameter.
Calculating Overall Diameter
Sidewall (inches) = Width (mm) × Aspect ratio ÷ 100 ÷ 25.4
Overall diameter = Wheel diameter + 2 × Sidewall
Circumference = Diameter × π Revolutions per mile = 63,360 ÷ Circumference
For 225/50R17: sidewall = 225 × 0.50 ÷ 25.4 = 4.43″, so the overall diameter is 17 + 8.86 = 25.86″.
The ±3% Rule
The accepted guideline is to keep the new overall diameter within 3% of the original. Beyond that, several systems begin to misbehave:
- Speedometer and odometer read proportionally wrong, and the error compounds over the life of the car.
- ABS and stability control infer wheel speed from rotation and can misjudge slip.
- Transmission shift points on many automatics are set from road speed.
- Clearance at full lock and full suspension compression can disappear, causing rubbing.
- Gearing effectively changes: a taller tire raises the final drive ratio, blunting acceleration and slightly lowering cruising revs.
Plus Sizing
Plus sizing fits a larger wheel while keeping overall diameter almost unchanged, by reducing the aspect ratio to compensate. Each inch of wheel diameter is offset by roughly 10 points of aspect ratio at typical widths.
| Step | Size | Overall diameter |
|---|---|---|
| Original | 205/65R15 | 25.49″ |
| Plus 1 | 215/55R16 | 25.31″ |
| Plus 2 | 225/45R17 | 24.97″ |
| Plus 3 | 245/40R18 | 25.72″ |
The trade-off is real: larger wheels with shorter sidewalls sharpen steering response and look better to most eyes, but ride harshness rises, wheel damage from potholes becomes far more likely, and unsprung weight usually increases.
Speedometer Error
Speed is inferred from wheel rotation, so a tire of a different circumference changes the reading proportionally:
Actual speed = Indicated speed × New diameter ÷ Original diameter
A 3% taller tire means an indicated 70 mph is really 72.1 mph. Note that most speedometers are already calibrated to read slightly high from the factory — European regulations require that they never read low — so a modestly taller tire can coincidentally improve accuracy.
Load Index and Speed Rating
After the size, the sidewall carries a load index and speed rating, such as 94V. Never fit a tire with a lower load index than the vehicle specifies.
| Speed rating | Maximum speed |
|---|---|
| S | 112 mph (180 km/h) |
| T | 118 mph (190 km/h) |
| H | 130 mph (210 km/h) |
| V | 149 mph (240 km/h) |
| W | 168 mph (270 km/h) |
| Y | 186 mph (300 km/h) |
Frequently Asked Questions
Can I fit wider tires on my existing wheels?
Usually by one step, if the rim width allows it. Each rim width supports a range of section widths, typically spanning about 20 mm. Going too wide for the rim distorts the tire's profile and hurts both handling and wear.
Will bigger tires hurt fuel economy?
Generally yes. Wider tires increase rolling resistance and aerodynamic drag, and larger wheels usually add unsprung weight. A significant size increase commonly costs 1–3 mpg.
Do all four tires have to match?
On the same axle, always. On all four, strongly recommended — and mandatory on all-wheel-drive vehicles, where mismatched rolling diameters can damage the differential or transfer case.
How do I read a P-metric or LT size?
A leading P means P-metric, for passenger cars; LT means light truck, with stiffer construction and higher load ratings. The three numbers are read the same way.