225/45R17 vs 225/40R18
Switching from 225/45R17 to 225/40R18 changes the overall diameter by +2.9 mm (+0.5%) — inside the commonly used ±3% guideline. Every figure below is calculated from the two size codes with the standard formulas.
| Measurement | 225/45R17 | 225/40R18 | Difference |
|---|---|---|---|
| Overall diameter | 634.3 mm | 637.2 mm | +2.9 mm (+0.5%) |
| Overall diameter | 24.97 in | 25.09 in | +0.11 in |
| Tire width | 225 mm (8.86 in) | 225 mm (8.86 in) | 0 mm |
| Sidewall height | 101.3 mm (3.99 in) | 90.0 mm (3.54 in) | -11.3 mm |
| Wheel (rim) diameter | 17.0 in | 18.0 in | +1.0 in |
| Circumference | 1993 mm | 2002 mm | +9 mm |
| Revolutions per mile | 2537 | 2526 | -12 |
| Revolutions per km | 502 | 500 | -2 |
Speedometer error
Calibrated for 225/45R17, the speedometer under-reads or over-reads once 225/40R18 is fitted. This is your real speed at each indicated speed:
| Speedometer shows | Actual speed on 225/40R18 | Error |
|---|---|---|
| 20 mph | 20.1 mph | +0.1 mph |
| 40 mph | 40.2 mph | +0.2 mph |
| 60 mph | 60.3 mph | +0.3 mph |
| 80 mph | 80.4 mph | +0.4 mph |
| 100 mph | 100.5 mph | +0.5 mph |
How these two sizes are worked out
225/45R17 breaks down as a 225 mm tread width with a sidewall of 45% of that width (101.3 mm) on a 17.0-inch wheel, giving an overall diameter of 634.3 mm (24.97 in). 225/40R18 works out to 637.2 mm (25.09 in) the same way. The standard formulas are the same ones used on the tire size calculator page: diameter = rim × 25.4 + 2 × sidewall, circumference = π × diameter, revolutions per mile = 63,360 ÷ diameter in inches.
Nominal sizes vary slightly between brands and models, and tread wear shrinks a tire over its life. These figures follow the size codes exactly; your vehicle manual or a professional tire shop is the authority for what fits your vehicle.
More comparisons
Frequently asked questions
Can I replace 225/45R17 tires with 225/40R18?
The diameter change is +0.5%, which is within the commonly used ±3% guideline. Diameter is only one part of fitment — wheel width range, load index, speed rating and physical clearance all have to check out too, so confirm with your vehicle manual or a professional tire shop before buying.
Will 225/40R18 make my speedometer read wrong?
Yes, by the diameter ratio. When the dial shows 60 mph you are really doing 60.3 mph on 225/40R18. The full table above covers 20 to 100 mph.
Which tire is taller, 225/45R17 or 225/40R18?
225/40R18 is taller — 637.2 mm against 634.3 mm overall diameter, a difference of 2.9 mm.