Tire size calculator
A tire comparison calculator: pick your current and new tire to see the difference in diameter, width and sidewall, the speedometer error and a drawing to scale.
+0.46%
Within 3%: generally safe to swap
| 225/45R17 | 225/40R18 | Difference | |
|---|---|---|---|
| Overall diameter | 24.97 in | 25.09 in | +0.11 in |
| Section width | 8.86 in | 8.86 in | 0.00 in |
| Sidewall height | 3.99 in | 3.54 in | -0.44 in |
| Circumference | 78.5 in | 78.8 in | +0.4 in |
| Revolutions per mile | 808 | 804 | -4 |
Speedometer error
- Speedometer shows 25
- 25.1 mph actual
- Speedometer shows 45
- 45.2 mph actual
- Speedometer shows 60
- 60.3 mph actual
- Speedometer shows 75
- 75.3 mph actual
Sizes within 3% of 225/45R17
How to compare tire sizes
- Find your current size on the tire’s sidewall or the placard inside the driver’s door, for example 225/45R17.
- Set it as the current tire, using the three menus or by typing the size.
- Set the size you’re considering as the new tire, or pick one from the list of sizes within 3%.
- Check the diameter difference and the speedometer error before you buy.
Wheel and tire size calculator: going up a wheel size
Bigger wheels need a lower-profile tire to keep the same overall diameter. Moving from 17-inch to 18-inch wheels, 225/45R17 becomes 225/40R18; to 19-inch wheels, 235/35R19. Pick your current size and the new wheel diameter in the calculator above, then choose a size from the list within 3%.
Not sure what the numbers on your tire mean? See how to read tire size.
How tire size is calculated
A metric tire size gives the width in millimeters, the aspect ratio and the wheel diameter in inches. The calculator works out the rest:
- Sidewall height = width × aspect ratio ÷ 100, converted to inches (÷ 25.4).
- Overall diameter = wheel diameter + 2 × sidewall height.
- Circumference = diameter × π.
- Revolutions per mile = 63,360 inches ÷ circumference.
Example: 225/45R17 has a sidewall of 225 × 0.45 = 101.25 mm (3.99 in), so its diameter is 17 + 2 × 3.99 = 24.97 inches and it turns about 808 times per mile.
Speedometer error explained
Your speedometer counts wheel revolutions and assumes the original tire size. A tire with a bigger diameter covers more road per turn, so you go faster than the speedometer says, and the odometer adds fewer miles than you drive. A smaller tire does the opposite. Within about 3% the difference is small and comparable to normal tire wear; beyond that, many shops recalibrate the speedometer.
Tire size comparison: what else changes
| Change | Effect |
|---|---|
| Taller tire | More ground clearance, speedometer reads low, slightly less acceleration, may rub. |
| Wider tire | More grip and a firmer look; more road noise and slightly worse fuel economy. |
| Lower aspect ratio | Sharper steering, firmer ride, more risk of rim damage on potholes. |
| Bigger wheel, same diameter | Plus sizing: sportier look and feel with the speedometer unchanged. |
Need the diameter of a common size at a glance? See the tire size chart.
Questions
How do I read a tire size like 225/45R17?
225 is the tire's width in millimeters, 45 is the sidewall height as a percentage of that width (45% of 225 mm, about 101 mm), R means radial construction, and 17 is the wheel diameter in inches.
How much bigger can I go with a different tire size?
The usual rule is to keep the overall diameter within 3% of the original. Beyond that, the speedometer, odometer, ABS and traction control read wrong, and the tire may rub on the fender liner or suspension.
Will a bigger tire change my speedometer?
Yes. A taller tire covers more ground per turn, so the speedometer reads lower than your real speed. A 3% taller tire means you're doing about 61.8 mph when the speedometer shows 60.
What is plus sizing?
Plus sizing means fitting a larger wheel with a lower-profile tire so the overall diameter stays the same: for example 225/45R17 to 225/40R18. The calculator's alternatives list shows sizes like this within 3%.
Can I put wider tires on my stock wheels?
Usually one size up (10 mm) is fine, but each wheel width has a range of approved tire widths. Check the tire maker's approved rim widths and that the wider tire clears the suspension at full lock.
