Tire size calculator
Diameter, width and sidewall in inches from a tire size, plus the speedometer error against another size.
- ๐บ๐ธ US federal & state rules
Diameter difference, 225/65R17 to 235/65R17
+1.79%
Within the 3% range most tire shops go by
- Speedometer reads 60 mph, you drive
- 61.1 mph
- Diameter
- 28.52 in to 29.03 in (+0.51 in)
- Ground clearance change
- +0.26 in
- Revolutions per mile
- 707 to 695
Calculated from the size code. Real tires vary slightly by brand.
Side by side
| Overall diameter | 28.52 in vs 29.03 in |
|---|---|
| Section width | 8.86 in vs 9.25 in |
| Sidewall height | 5.76 in vs 6.01 in |
| Wheel diameter | 17 in vs 17 in |
| Circumference | 89.58 in vs 91.19 in |
| Revolutions per mile | 707 vs 695 |
Speedometer vs actual speed
| Shows 25 mph | 25.4 mph |
|---|---|
| Shows 35 mph | 35.6 mph |
| Shows 45 mph | 45.8 mph |
| Shows 55 mph | 56 mph |
| Shows 65 mph | 66.2 mph |
| Shows 75 mph | 76.3 mph |
- Taller tires make the speedometer read low, so you go faster than it shows. Shorter tires make it read high. The odometer is off by the same percent.
- The 3% limit is a common shop rule, not a law. Check the sizes your vehicle maker approves on the door jamb placard and make sure the new tire clears the fenders and suspension.
- Match or exceed the original load index and speed rating.
Show the math
| Sidewall | width ร aspect ratio รท 25.4 = 5.76 in |
|---|---|
| Diameter | 17 + 2 ร 5.758 = 28.52 in |
| Revolutions per mile | 63,360 รท (ฯ ร 28.516) = 707.3 |
| New diameter | 29.03 in |
| Difference | 29.028 รท 28.516 minus 1 = +1.795% |
| Actual speed | 60 ร 1.0179 = 61.1 mph |
What to do next
How it works
For a P-metric size, sidewall height = width ร aspect ratio, converted from millimeters to inches by dividing by 25.4. Overall diameter = wheel size + two sidewalls. Revolutions per mile = 63,360 inches รท (ฯ ร diameter).
Flotation sizes such as 33x12.50R17 already give the diameter (33 inches) and width (12.5 inches), so the sidewall is (33 minus 17) รท 2 = 8 inches.
Things to keep in mind
Your speedometer counts wheel turns. A taller tire covers more ground per turn, so the car moves faster than the gauge shows, and the odometer adds fewer miles than you really drive.
Changing size can also affect ABS, traction control and transmission shift points on some vehicles. Stick to sizes listed for your vehicle when you can.
Frequently asked questions
How do I read a tire size like 225/65R17?
225 is the width in millimeters, 65 means the sidewall is 65% of that width, R is radial and 17 is the wheel diameter in inches. That tire is about 28.5 inches tall.
How tall is a 265/70R17 tire?
The sidewall is 265 ร 0.70 = 185.5 mm, or 7.30 inches. Two sidewalls plus the 17 inch wheel make about 31.6 inches.
How much does a bigger tire change my speedometer?
By the same percent as the diameter. A tire 3% taller means the speedometer shows 60 mph when you are going about 61.8 mph.
How far can I go from the stock tire size?
Tire shops commonly stay within 3% of the original diameter. Your vehicle maker's approved sizes and the clearance in your wheel wells matter more than any rule of thumb.