Tire Size Calculator & Comparison
Compare any two tire sizes: diameter, sidewall, circumference and the speedometer error a swap creates — with the ±3% fitment verdict.
The industry rule of thumb is ±3% overall diameter: beyond it the speedometer, odometer, ABS thresholds and effective gearing all shift noticeably. Plus-sizing done right (wider tire, lower profile, bigger rim) keeps diameter nearly constant on purpose.
Formula
⚠️ Estimates for planning and education — verify fitment, gearing and speeds against manufacturer data and local law. Never test results on public roads.
Compare any two tire sizes: diameter, sidewall, circumference and the speedometer error a swap creates — with the ±3% fitment verdict.
About Tire Size Calculator & Comparison
Every tire swap is three numbers pretending to be one: section width in millimetres, sidewall as a percentage of that width, and rim diameter in inches — a metric-imperial sandwich that makes overall diameter anything but obvious. This calculator decodes both sizes, compares diameter, sidewall and circumference, computes the speedometer error the difference creates, and applies the ±3% fitment guideline that tire shops, ABS engineers and lease inspectors all quietly use.
How to use Tire Size Calculator & Comparison
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula diameter (in) = rim + 2 × (width × aspect/100) / 25.4; error % = (new − old) / old × 100 substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Tire Size Calculator & Comparison?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula diameter (in) = rim + 2 × (width × aspect/100) / 25.4; error % = (new − old) / old × 100 with sources cited on the page
- ✓The industry rule of thumb is ±3% overall diameter: beyond it the speedometer, odometer, ABS thresholds and effective gearing all shift noticeably. Plus-sizing done right (wider tire, lower profile, bigger rim) keeps diameter nearly constant on purpose.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why does the ±3% diameter rule exist?+
Because several systems silently assume the factory rolling circumference: the speedometer and odometer count driveshaft or wheel-speed pulses, ABS and stability control compare wheel speeds against thresholds, and automatic transmissions pick shift points from road speed. A 3% taller tire under-reads the speedo about 3%, stretches effective gearing the same amount, and starts to confuse systems calibrated to tighter tolerances — most manufacturers and tire associations draw the line there.
What is plus-sizing and why does it preserve diameter?+
Plus-one or plus-two sizing raises rim diameter while widening the tire and shrinking the aspect ratio so overall diameter barely moves: 225/45R17 → 245/40R18 changes diameter only about 0.4%. Handling sharpens (shorter, stiffer sidewall), the speedometer stays honest, and gearing is untouched — which is exactly why factory option packages use it. The cost: harsher ride, heavier wheels and more pothole vulnerability.
Does a taller tire make my gearing shorter or taller?+
Taller (numerically lower effective ratio): the wheel covers more ground per revolution, so engine RPM at any speed drops by the same percentage the diameter grew. A 3% taller tire is like swapping a 3.73 axle for a 3.62 — slightly lazier acceleration, slightly lower cruise RPM. Off-roaders who fit 35-inch tires usually re-gear the axles to claw the lost crawl ratio back; our re-gear calculator does that math.
Will the wider tire fit my wheel and fender?+
Diameter math says nothing about clearance: width is limited by rim width range (each tire size lists an approved span — roughly width/25.4 minus 1 to plus 1 inch), suspension travel, and steering lock. Check the tire maker's approved-rim table and measure clearance at full lock and full compression; rubbing at the fender liner on one side is the classic plus-size complaint the calculator can't predict.
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