Methodology
How the comparison works.
Compare a current tire with a proposed tire. Calculations retain full precision and round for display. These are dimensional estimates, not a vehicle-fitment decision.
Nominal dimensions
For metric sizes, sidewall height is section width × aspect ratio ÷ 100. Diameter is two sidewalls plus rim diameter × 25.4. For flotation sizes such as 35x12.50R17, the first number supplies overall diameter in inches directly; the second supplies width. The interactive calculator also supports the 16.5, 17.5, 19.5 and 22.5-inch LT and commercial rims; the reviewed page catalog uses its narrower, whole-inch metric size contract.
Geometric circumference is diameter × pi. Nominal geometric revolutions per mile is 1,609,344 ÷ circumference in millimeters. This is not a measured rolling value.
Optional supplied measurements
You can enter an exact tire model's published overall diameter, measured section width and its measuring rim, and revolutions per mile. These entries are labeled user supplied and unverified, including any source description. They travel with the comparison link and swap with the tire. Changing the tire size clears its measurements.
A supplied overall diameter replaces nominal diameter for dimensional comparisons. Radial sidewall height remains an estimate of half the difference between overall diameter and rim diameter. A supplied section width affects tire-position estimates only when its measuring rim equals the entered wheel width. Otherwise nominal section width is used with an explanation. No universal wheel-width correction is applied.
Tire Rack explains how measuring-rim width affects actual section width. Check the exact product specifications and approved rim range.
Estimated speed and ride height
With diameter data, estimated actual speed = indicated speed × proposed diameter ÷ current diameter. If you supply revolutions per mile for both tires, speed instead uses current rev/mile ÷ proposed rev/mile. A single supplied rolling value is never mixed with a geometric estimate. The result states which basis is used.
Both approaches assume the speedometer was accurate with the current tires. Actual rolling behavior varies with load, pressure, speed, wear, and tire construction. Michelin defines measured revolutions per mile and rolling circumference. Nominal ride-height change is half the diameter difference and does not model loaded suspension behavior.
Wheel and tire positions
Without wheel information, only tire results appear. “Keeping the same wheels” requires matching rim diameters and calculates relative tire-edge changes from half the nominal section-width difference. It does not invent an offset or draw an absolute mounting-plane diagram.
When you enter both actual wheel setups, outer wheel position is half the bead-seat width minus offset; inner position is half the bead-seat width plus offset. Tire estimates use half the applicable section width with the same offset signs. All deltas are proposed minus current. A positive outer delta moves toward the fender; a positive inner delta moves toward the suspension. These describe movement, not remaining clearance. Wheel flange shape and actual mounted dimensions are not modeled.
Compatibility remains unverified
The 3% and 5% diameter bands determine the strength of comparison guidance. They are not universal vehicle allowances. A low variance does not establish clearance, load capacity, brake compatibility, or suitability for an AWD or staggered setup. The physical-check list describes checks still to do.
USTMA replacement guidance explains why vehicle-manufacturer requirements matter.
Catalog evidence
Published pages use an explicitly reviewed allowlist. Historical size evidence comes from the NHTSA Consumer Complaints archive, listed on NHTSA's datasets and APIs page (direct ZIP download, about 373 MB), and its data dictionary. The August 19, 2026 snapshot has no accepted populated tire-size records later than May 13, 2021. Counts establish historical appearances, not popularity, safety, or current availability.
Current manufacturer size listings are also accepted, with an exact source URL, publisher, verification date, and review scope. The September 5, 2026 additions use Michelin's 20-inch, 21-inch, and 22-inch listings. A listed size is not proof of dealer stock or a manufacturer-measured dimension.
What a dimension review means
A published pair must meet the same-rim, plus-one, or plus-two relationship rules, remain within 3% nominal diameter variance, carry a dated review decision, and connect to at least four related comparisons. Review confirms the page's dimensional and evidence requirements only. Expansion addresses documented coverage gaps or observed demand; it is never automatic fitment approval.
Read the editorial policy, understand the tool's limits, or open the calculator.
Last updated: .