How the calculator works it out
A metric tire size gives you the width in millimeters, the sidewall height as a percentage of that width, and the wheel size in inches. The overall diameter is the wheel plus two sidewalls. Everything else follows from the diameter.
Sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4
Diameter (in) = wheel (in) + 2 × sidewall
Circumference = 3.1416 × diameter · Revs per mile = 63,360 ÷ circumference (in)
Actual speed = speedometer reading × new diameter ÷ old diameter
Worked example: 225/65R17 to 235/65R17
The current sidewall is 225 × 0.65 = 146.25 mm, which is 5.758 inches, so the tire is 17 + 11.516 = 28.52 inches tall. The new sidewall is 235 × 0.65 = 152.75 mm, or 6.014 inches, giving 29.03 inches. The new tire is 29.03 ÷ 28.52 = 1.018 times as tall, 1.8% bigger. When the speedometer shows 60 mph you are doing 60 × 1.018 = 61.1 mph. That is inside the 3% rule of thumb.
The 3% rule
Tire retailers commonly advise keeping the new overall diameter within 3% of the original. Discount Tire puts it this way: if the difference in height is within 3% of the factory height you are generally fine, but it is not a hard rule, and wheel well size, suspension, offset and wheel specs all matter. Tire Rack's plus-sizing guide does not give a percentage; it says to keep the overall diameter the same where possible, because large changes affect the speedometer, anti-lock brakes, traction control and stability control, as well as ground clearance and gearing. The calculator flags anything over 3% in either direction.
What a size change does
| Diameter change | Speedometer shows 60 mph, you are doing | Odometer after 10,000 real miles |
|---|---|---|
| −3% | 58.2 mph | 10,309 miles |
| −1% | 59.4 mph | 10,101 miles |
| +1% | 60.6 mph | 9,901 miles |
| +3% | 61.8 mph | 9,709 miles |
| +5% | 63.0 mph | 9,524 miles |
A taller tire also raises the effective gearing, so the engine turns slower at a given speed and acceleration feels softer. A shorter tire does the reverse. Many shops can recalibrate the speedometer after a big change.
Reading other size formats
- P-metric (P225/65R17) and plain metric (225/65R17) are read the same way here. Load ratings differ, so do not mix them on one axle without checking the load index.
- LT metric (LT265/70R17) is a light truck tire. The dimensions are calculated the same way.
- ZR (245/40ZR18) marks a high-speed rated tire; the dimensions are unaffected.
- Flotation (33x12.50R15) gives height × width in inches, then the wheel size.
Calculated sizes are nominal. Real tires of the same size vary by a few tenths of an inch between brands and models, so check the manufacturer's spec sheet if you are close to a clearance limit.
Questions people ask
How tall is a 225/65R17 tire?
About 28.5 inches. The sidewall is 65% of 225 mm, which is 146.25 mm or 5.76 inches. There are two sidewalls, one above and one below the 17 inch wheel, so 17 + 2 × 5.76 = 28.52 inches overall.
How much bigger can I go on tire size?
The common advice from tire retailers such as Discount Tire is to keep the new overall diameter within 3% of the original. That is a rule of thumb, not a law, and fitment also depends on wheel well space, suspension and wheel offset. Some vehicles have less room than 3%, so check that the tire clears at full steering lock and suspension compression.
Will bigger tires make my speedometer read wrong?
Yes. A taller tire covers more ground per turn, so you travel faster than the speedometer shows. With tires 3% taller, an indicated 60 mph is really about 61.8 mph. Smaller tires do the opposite and make the speedometer read high. The odometer is off by the same percentage.
What do the numbers in a tire size mean?
In 225/65R17, 225 is the section width in millimeters, 65 is the sidewall height as a percentage of that width, R means radial construction and 17 is the wheel diameter in inches. A leading P means a passenger tire and LT means light truck. Numbers after the size, such as 102H, are the load index and speed rating.
How do flotation sizes like 33x12.50R15 work?
Flotation sizes, common on off-road trucks, give the overall diameter first: 33 inches tall, 12.5 inches wide, on a 15 inch wheel. The sidewall is (33 − 15) ÷ 2 = 9 inches. The calculator accepts these and can compare them with metric sizes.
Why do the revolutions per mile not match the tire maker's spec sheet?
This calculator uses the geometric diameter of an unloaded tire. Under the weight of a car the tire flattens slightly where it meets the road, so it turns a little more per mile. Manufacturers measure revolutions per mile on a loaded tire, so their figure usually comes out somewhat higher.