275/55R20 vs 285/50R20
Switching from 275/55R20 to 285/50R20 changes the overall diameter by -17.5 mm (-2.2%) — inside the commonly used ±3% guideline. Every figure below is calculated from the two size codes with the standard formulas.
| Measurement | 275/55R20 | 285/50R20 | Difference |
|---|---|---|---|
| Overall diameter | 810.5 mm | 793.0 mm | -17.5 mm (-2.2%) |
| Overall diameter | 31.91 in | 31.22 in | -0.69 in |
| Tire width | 275 mm (10.83 in) | 285 mm (11.22 in) | +10 mm |
| Sidewall height | 151.3 mm (5.95 in) | 142.5 mm (5.61 in) | -8.8 mm |
| Wheel (rim) diameter | 20.0 in | 20.0 in | 0.0 in |
| Circumference | 2546 mm | 2491 mm | -55 mm |
| Revolutions per mile | 1986 | 2029 | +44 |
| Revolutions per km | 393 | 401 | +9 |
Speedometer error
Calibrated for 275/55R20, the speedometer under-reads or over-reads once 285/50R20 is fitted. This is your real speed at each indicated speed:
| Speedometer shows | Actual speed on 285/50R20 | Error |
|---|---|---|
| 20 mph | 19.6 mph | -0.4 mph |
| 40 mph | 39.1 mph | -0.9 mph |
| 60 mph | 58.7 mph | -1.3 mph |
| 80 mph | 78.3 mph | -1.7 mph |
| 100 mph | 97.8 mph | -2.2 mph |
How these two sizes are worked out
275/55R20 breaks down as a 275 mm tread width with a sidewall of 55% of that width (151.3 mm) on a 20.0-inch wheel, giving an overall diameter of 810.5 mm (31.91 in). 285/50R20 works out to 793.0 mm (31.22 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 275/55R20 tires with 285/50R20?
The diameter change is -2.2%, 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 285/50R20 make my speedometer read wrong?
Yes, by the diameter ratio. When the dial shows 60 mph you are really doing 58.7 mph on 285/50R20. The full table above covers 20 to 100 mph.
Which tire is taller, 275/55R20 or 285/50R20?
275/55R20 is taller — 810.5 mm against 793.0 mm overall diameter, a difference of 17.5 mm.