Tire Size Calculator
The tire size calculator compares two sizes by diameter, sidewall and circumference, with the speedometer and odometer error and whether the change is inside the 3% tolerance.
Notice: For general informational purposes only. A change in tire size can affect braking, handling, clearance and speedometer accuracy, and an improper fitment can cause loss of vehicle control. Confirm fitment against the vehicle manufacturer's specifications and with a qualified tire professional.
Diameter change
- Diameter change Within the conventional 3% tolerance.
- +0.21%
- Current tire Sidewall 146.3 mm (5.76"), circumference 89.58".
- 225/65R17, 28.52" diameter
- New tire Sidewall 147 mm (5.79"), circumference 89.77".
- 245/60R17, 28.57" diameter
- When your speedometer reads 70 mph A larger tire covers more ground per revolution than the speedometer was calibrated for, so you are going FASTER than indicated.
- you are actually doing 70.1 mph
- Odometer error Over 10,000 indicated miles you would actually travel 10,021.
- 0.21% under-reads
- Revolutions per mile 1 fewer per mile on the new size.
- 707 becomes 706
- Ground clearance change Half the diameter change, since the axle sits at the center.
- +0.03"
- Width change
- +20 mm
| Indicated | Actual speed | Difference |
|---|---|---|
| 25 mph | 25.1 mph | +0.1 mph |
| 35 mph | 35.1 mph | +0.1 mph |
| 55 mph | 55.1 mph | +0.1 mph |
| 65 mph | 65.1 mph | +0.1 mph |
| 75 mph | 75.2 mph | +0.2 mph |
How the Tire Size Calculator works
A tire size like 225/65R17 packs three measurements into one string, in two different unit systems, and the middle one is a ratio rather than a length. That is the whole reason the arithmetic is non-obvious.
225 is the section width in MILLIMETERS. 65 is the aspect ratio: the sidewall height as a PERCENTAGE of the width, so 65% of 225 mm = 146.25 mm. R means radial construction. 17 is the wheel diameter in INCHES.
Overall diameter is the wheel plus two sidewalls, which needs the millimeter sidewall converted: 146.25 mm is 5.76 inches, so the tire is 17 + 11.52 = 28.52 inches tall.
Diameter is what matters for everything downstream, because the speedometer counts wheel revolutions and multiplies by an assumed circumference. Fit a tire 3% larger and the car travels 3% further per revolution than the speedometer assumes, so at an indicated 70 mph you are actually doing 72.1. The odometer is off by the same share: 10,000 indicated miles is 10,300 actual.
The conventional tolerance is 3%, a convention rather than a regulation. Within it, speedometer error stays under about 2 mph at highway speed and the vehicle's electronics generally cope. Beyond it, several things that are not obvious start to matter: ABS and stability control infer wheel slip from rotational speed and are calibrated to the original rolling diameter, automatic transmissions use road speed for shift points, and tire pressure monitoring can be affected. A substantially larger tire is more than a speedometer problem.
Three physical checks that no calculator can do for you. Width clearance to the strut and the inner fender, especially at full steering lock, where rubbing shows up first. Load index: a larger tire is not automatically rated for more weight, and fitting one below the vehicle's placard rating is unsafe. And speed rating, which should meet or exceed the original.
On the trade-offs, since this is usually a choice rather than a calculation. A lower aspect ratio (a shorter sidewall on a bigger wheel) gives sharper steering response and less sidewall flex, at the cost of ride comfort, wheel damage resistance on potholes, and usually price. A taller sidewall absorbs impacts, is cheaper, and is more forgiving of bad roads. The move toward ever-larger wheels is substantially aesthetic, and the ride penalty is real.
One practical note: the placard in the driver's door jamb gives the original size and the correct pressures for it. If you change size, the pressure that was right for the original is not necessarily right for the new one, since load capacity at a given pressure depends on the tire's air volume.
Tire Size Calculator FAQ
How do I read a tire size?
225/65R17 means 225 mm section width, a sidewall 65% of that width, radial construction, and a 17-inch wheel. The overall diameter is the wheel plus two sidewalls: 17 + 2 x (225 x 0.65 / 25.4) = 28.5 inches.
How much bigger can my tires be?
The conventional limit is 3% overall diameter. Within that, speedometer error stays under about 2 mph at highway speed and the vehicle's ABS, stability control and transmission calibration generally cope. Also check clearance at full steering lock and that the load index meets the door placard.
Do bigger tires make my speedometer wrong?
Yes. The speedometer counts wheel revolutions and assumes the original circumference, so a 3% larger tire means you are traveling 3% faster than indicated: 72 mph when it reads 70. The odometer under-reads by the same proportion, which affects service intervals and resale mileage.
Does a wider tire change the diameter?
At the same aspect ratio, yes, because the ratio is a percentage of the width. 245/65R17 is taller than 225/65R17 since 65% of 245 exceeds 65% of 225. To widen without growing, drop the aspect ratio; 245/60R17 is close to the original height.