275/65R18 vs 33x12.50R18
Switching from 275/65R18 to 33x12.50R18 changes the overall diameter by +23.5 mm (+2.9%) — inside the commonly used ±3% guideline. Every figure below is calculated from the two size codes with the standard formulas.
| Measurement | 275/65R18 | 33x12.50R18 | Difference |
|---|---|---|---|
| Overall diameter | 814.7 mm | 838.2 mm | +23.5 mm (+2.9%) |
| Overall diameter | 32.07 in | 33.00 in | +0.93 in |
| Tire width | 275 mm (10.83 in) | 318 mm (12.50 in) | +43 mm |
| Sidewall height | 178.8 mm (7.04 in) | 190.5 mm (7.50 in) | +11.8 mm |
| Wheel (rim) diameter | 18.0 in | 18.0 in | 0.0 in |
| Circumference | 2559 mm | 2633 mm | +74 mm |
| Revolutions per mile | 1975 | 1920 | -55 |
| Revolutions per km | 391 | 380 | -11 |
Speedometer error
Calibrated for 275/65R18, the speedometer under-reads or over-reads once 33x12.50R18 is fitted. This is your real speed at each indicated speed:
| Speedometer shows | Actual speed on 33x12.50R18 | Error |
|---|---|---|
| 20 mph | 20.6 mph | +0.6 mph |
| 40 mph | 41.2 mph | +1.2 mph |
| 60 mph | 61.7 mph | +1.7 mph |
| 80 mph | 82.3 mph | +2.3 mph |
| 100 mph | 102.9 mph | +2.9 mph |
How these two sizes are worked out
275/65R18 breaks down as a 275 mm tread width with a sidewall of 65% of that width (178.8 mm) on a 18.0-inch wheel, giving an overall diameter of 814.7 mm (32.07 in). 33x12.50R18 works out to 838.2 mm (33.00 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 275/65R18 tires with 33x12.50R18?
The diameter change is +2.9%, 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 33x12.50R18 make my speedometer read wrong?
Yes, by the diameter ratio. When the dial shows 60 mph you are really doing 61.7 mph on 33x12.50R18. The full table above covers 20 to 100 mph.
Which tire is taller, 275/65R18 or 33x12.50R18?
33x12.50R18 is taller — 838.2 mm against 814.7 mm overall diameter, a difference of 23.5 mm.