Bigger tire,honest numbers.

Reading a tire size

Take 275/60R20. The 275 is section width in millimetres. The 60is the aspect ratio — sidewall height as a percentage of that width, so this sidewall is 165 mm tall. The R means radial construction, and 20 is the rim diameter in inches.

Overall diameter is rim diameter plus two sidewalls: 20″ + 2 × 6.50″ = 33.0″. That is the number the calculator works from, because it is the number your speedometer, transmission, and stability control care about. Section width barely matters to any of them.

The 3% rule

Stay within roughly 3% of stock overall diameter and your speedometer, shift points, and ABS calibration stay accurate enough that nothing needs touching. Past 3%, real things start happening:

• Speedometer under-reads. A taller tire covers more ground per revolution, so the vehicle thinks it is going slower than it is. The calculator shows exactly what your gauge will read at a true 70 mph.
• Odometer under-counts. Which flatters your service intervals and quietly costs you warranty coverage measured in miles.
• Effective gearing changes. A taller tire behaves like a taller final drive: later shifts, more torque converter slip, worse towing behaviour on grades. This is the change people feel first.
• Stability control works from bad data. Traction and stability systems compare wheel speeds against expected values. Big diameter changes shift what "expected" means.

Most vehicles can be recalibrated, some from the factory scan tool and some with a third-party module. It is worth budgeting for if you are going more than 3% over — we will tell you which camp your vehicle is in before you order.

Revolutions per mile, and why it is on the readout

Revs per mile is the cleanest single expression of a diameter change: a smaller tire spins more times to cover a mile. It is the number that drives speedometer error and effective gearing, and on all-wheel-drive vehicles it is the number that matters most, because mismatched revs per mile front to rear puts a constant load through the centre differential or transfer case. Never mix diameters across axles on an AWD vehicle, and replace all four at once when tread depth differs significantly.

Sidewall is ride quality

A 20″ wheel with a 55-series tire and a 22″ wheel with a 40-series tire can share an overall diameter and ride nothing alike. Sidewall is the only suspension component between your wheel and the road, and shorter sidewalls transmit more impact, are far easier to curb, and make wheel damage from potholes much more likely. If you drive Salt Lake County streets in March, that is a real consideration.

Load rating, and winter

On trucks, load range is not optional. If you tow or haul, the tire has to carry it — and an E-rated tire rides noticeably stiffer when the bed is empty, which surprises people who tow twice a year. Tell us your actual use and we will size the load range to it rather than to the worst case.

Separately: all-season compounds harden below about 45 °F. If you drive the canyons in January, a dedicated winter set on a second set of wheels is the cheapest safety upgrade available, and swapping wheel-and-tire assemblies twice a year costs less in labour than remounting tires.

Once the diameter works, check how the wheel itself will sit with thewheel offset visualizer, then see package pricing on thewheels and tires page.