Tire & Wheel Calculator

"Plus-sizing" (+1, +2, +3) emerged as a tire-shop marketing concept in the 1980s. Car magazines popularized the idea: keep the overall tire diameter constant while increasing wheel size by 1 inch and reducing sidewall aspect ratio proportionally. A 205/65R15 (25.5" OD) replaced with 215/55R16 (25.3") is a "Plus-1" upgrade; 225/45R17 (25.0") is Plus-2.

The performance argument: stiffer sidewalls (shorter, wider contact patch, less deflection under load) give crisper steering response and less body roll through corners. The cost: rougher ride, more susceptibility to pothole damage, higher wheel/tire replacement expense, and heavier unsprung mass. The trend peaked around 2010-2015 on sedans and SUVs; EV-era priorities (quietness, range efficiency) are reversing it.

Speedometer error compounds over mileage: a 5% oversized tire means your odometer reads 95,000 miles when the car has actually traveled 100,000 miles. Lease returns, emission-warranty windows, and service-interval timers all get shifted accordingly. Some states (and rental companies) require tire diameter changes to be noted on title or keep stock tires at all times.

Enter stock tire dimensions (width, aspect, wheel diameter) and new tire dimensions. The tool computes overall diameter change (inches and %), speedometer error (indicated vs actual speed), revolutions-per-mile change, and sidewall-height difference. A negative % means the new tire is smaller (car actually goes slower than speedometer shows).

Stay within ±3% overall diameter for street use. For off-road builds that exceed 3%, reprogram the speedometer (OBD-II tuners) or regear the axles to compensate. Also check inner-fender clearance at full steering lock and full suspension compression before committing to the larger tire. Everything runs client-side; no values leave your browser.