225/50R17 vs 245/45R18
Switching from 225/50R17 to 245/45R18 changes the overall diameter by +20.9 mm (+3.2%) — outside the commonly used ±3% guideline. Every figure below is calculated from the two size codes with the standard formulas.
| Measurement | 225/50R17 | 245/45R18 | Difference |
|---|---|---|---|
| Overall diameter | 656.8 mm | 677.7 mm | +20.9 mm (+3.2%) |
| Overall diameter | 25.86 in | 26.68 in | +0.82 in |
| Tire width | 225 mm (8.86 in) | 245 mm (9.65 in) | +20 mm |
| Sidewall height | 112.5 mm (4.43 in) | 110.3 mm (4.34 in) | -2.3 mm |
| Wheel (rim) diameter | 17.0 in | 18.0 in | +1.0 in |
| Circumference | 2063 mm | 2129 mm | +66 mm |
| Revolutions per mile | 2450 | 2375 | -76 |
| Revolutions per km | 485 | 470 | -15 |
Speedometer error
Calibrated for 225/50R17, the speedometer under-reads or over-reads once 245/45R18 is fitted. This is your real speed at each indicated speed:
| Speedometer shows | Actual speed on 245/45R18 | Error |
|---|---|---|
| 20 mph | 20.6 mph | +0.6 mph |
| 40 mph | 41.3 mph | +1.3 mph |
| 60 mph | 61.9 mph | +1.9 mph |
| 80 mph | 82.5 mph | +2.5 mph |
| 100 mph | 103.2 mph | +3.2 mph |
How these two sizes are worked out
225/50R17 breaks down as a 225 mm tread width with a sidewall of 50% of that width (112.5 mm) on a 17.0-inch wheel, giving an overall diameter of 656.8 mm (25.86 in). 245/45R18 works out to 677.7 mm (26.68 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 225/50R17 tires with 245/45R18?
The diameter change is +3.2%, which is outside 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 245/45R18 make my speedometer read wrong?
Yes, by the diameter ratio. When the dial shows 60 mph you are really doing 61.9 mph on 245/45R18. The full table above covers 20 to 100 mph.
Which tire is taller, 225/50R17 or 245/45R18?
245/45R18 is taller — 677.7 mm against 656.8 mm overall diameter, a difference of 20.9 mm.