Tire & Wheel Calculator
Compare two tire sizes side-by-side — diameter difference, speedometer error, clearance.
How to Use
- Enter your stock and new tire sizes (width / aspect / wheel).
- See both tires drawn to scale with the diameter change, speedometer error, and revs-per-mile shift.
- Over 3%? Recalibrate the speedo or compensate with the Gear Ratio calculator.
Show Work
Key outputs
History of Plus-Sizing
"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.
About This Calculator
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.
About the Tire & Wheel Calculator
The Tire & Wheel Calculator gives you a fast, free answer for automotive and vehicle projects without sending anything off your device. Compare two tire sizes side-by-side — diameter difference, speedometer error, clearance.
How it works
Enter your figures and the result appears instantly, updating the moment you change anything. There is no submit button and nothing to wait for, so it is easy to try a few what-if numbers and compare the results. Just check each box holds the kind of value it expects.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
How much size change is safe?
Within 3% overall diameter to avoid significant speedometer error. Beyond that, consider a speedometer recalibration or gearing change.
How do I use the Tire & Wheel Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Plus-size
Going from 17" to 18" wheels with matching tire.
Off-road upsize
Going to bigger tires for clearance.
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