Compare tire sizes, calculate speedometer differences, and find plus-size fitments.
Enter your current tire and a candidate size in the standard code, such as 225/45R17, and the tool decodes both: section width in millimeters, sidewall height as a percentage of that width, and rim diameter in inches. It reconstructs each tire's overall diameter - two sidewalls plus the rim - and converts the difference into a percent change, a speedometer correction, and a plus-size fitment match. The usual surprise: the rim size printed on the wheel is nearly irrelevant by itself. What changes your speedometer and ride height is the full tire diameter.
A concrete input and expected output from the current implementation.
Input
Stock: 205/55R16, new: 225/60R16
Expected output
Stock diameter 24.88 in; new diameter 26.63 in (+7.0%). At a gauge reading of 60 mph you are actually traveling about 64.2 mph. The car sits about 0.9 in (22 mm) higher, and the tire rolls 7% farther per revolution.
Sidewall height is width times aspect ratio: 205 x 0.55 = 112.75 mm (4.44 in), so stock diameter is 16 + 2 x 4.44 = 24.88 in. The new sidewall is 225 x 0.60 = 135 mm (5.31 in), giving 16 + 2 x 5.31 = 26.63 in. Actual speed scales by the diameter ratio: 60 x 26.63 / 24.88 = 64.2 mph.
A size like 225/45R17 means: 225 mm section width, sidewall height equal to 45% of that width, and a 17-inch rim. Sidewall height is 225 x 0.45 = 101.25 mm = 3.99 in, so overall diameter is 17 + 2 x 3.99 = 24.97 in. The tool does this for both tires, because overall diameter - not rim size - drives circumference, ride height, and speedometer error.
The gauge is calibrated for a stock rolling circumference. If the new tire's diameter differs by a factor k, the car travels k times farther per revolution than the gauge assumes, so actual speed equals indicated speed times k. A 7% larger tire means 64 mph at a gauge reading of 60; a smaller tire does the reverse. Figures assume new tires at nominal size; brand, pressure, load, and tread wear move the real number.
Hardly at all. The stock tire measures 24.88 in across and the new one 24.97 in - a 0.4% increase - so at a gauge reading of 60 mph you are actually doing about 60.2 mph. That is the point of plus sizing: as the rim grows, the aspect ratio drops to keep overall diameter almost unchanged. An error this small is inside normal speedometer tolerance.
Arithmetically yes - a larger diameter makes the gauge read lower - but it is usually the wrong fix. Correcting a 5% gauge error needs a tire 5% larger, beyond the roughly 3% diameter change commonly cited as safe, and it also slows acceleration, raises the car, and risks rubbing. Check the gauge against GPS first; a slightly high reading is common by design.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
Create customizable checklists with groups, due dates, and priorities for any project.
Design a printable daily planner with time blocks, goals, and priority sections.
Measure your typing speed in WPM and accuracy with real-time feedback and stats.
Pomodoro timer with built-in ambient noise (rain, cafe, forest) generated via Web Audio API oscillators.
Stay focused with a customizable Pomodoro timer featuring work and break intervals.
A precise stopwatch with lap tracking, split times, and exportable results.