225/55R17 vs 235/50R18
Switching from 225/55R17 to 235/50R18 changes the overall diameter by +12.9 mm (+1.9%) — inside the commonly used ±3% guideline. Every figure below is calculated from the two size codes with the standard formulas.
| Measurement | 225/55R17 | 235/50R18 | Difference |
|---|---|---|---|
| Overall diameter | 679.3 mm | 692.2 mm | +12.9 mm (+1.9%) |
| Overall diameter | 26.74 in | 27.25 in | +0.51 in |
| Tire width | 225 mm (8.86 in) | 235 mm (9.25 in) | +10 mm |
| Sidewall height | 123.8 mm (4.87 in) | 117.5 mm (4.63 in) | -6.3 mm |
| Wheel (rim) diameter | 17.0 in | 18.0 in | +1.0 in |
| Circumference | 2134 mm | 2175 mm | +41 mm |
| Revolutions per mile | 2369 | 2325 | -44 |
| Revolutions per km | 469 | 460 | -9 |
Speedometer error
Calibrated for 225/55R17, the speedometer under-reads or over-reads once 235/50R18 is fitted. This is your real speed at each indicated speed:
| Speedometer shows | Actual speed on 235/50R18 | Error |
|---|---|---|
| 20 mph | 20.4 mph | +0.4 mph |
| 40 mph | 40.8 mph | +0.8 mph |
| 60 mph | 61.1 mph | +1.1 mph |
| 80 mph | 81.5 mph | +1.5 mph |
| 100 mph | 101.9 mph | +1.9 mph |
How these two sizes are worked out
225/55R17 breaks down as a 225 mm tread width with a sidewall of 55% of that width (123.8 mm) on a 17.0-inch wheel, giving an overall diameter of 679.3 mm (26.74 in). 235/50R18 works out to 692.2 mm (27.25 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 225/55R17 tires with 235/50R18?
The diameter change is +1.9%, 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 235/50R18 make my speedometer read wrong?
Yes, by the diameter ratio. When the dial shows 60 mph you are really doing 61.1 mph on 235/50R18. The full table above covers 20 to 100 mph.
Which tire is taller, 225/55R17 or 235/50R18?
235/50R18 is taller — 692.2 mm against 679.3 mm overall diameter, a difference of 12.9 mm.