Tire Size & Speedometer Error Calculator

Overall diameter change, true vs indicated speed, and revolutions per mile from two tire size codes

Rolling-diameter math — not a structural-safety, load, or rubbing certificate. Overall diameter comes from the size code: twice the sidewall (width × aspect ratio) plus the rim. Speedometer and odometer error follow the circumference change. The ±3% band is common shop fitment guidance for staying close to the original rolling diameter without a cluster recalibration. It is not a guarantee of fender clearance, load index, speed rating, TPMS, or AWD/4WD compatibility. Confirm the door-sticker load/speed ratings and a real fitment check.

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Tire Size Change Gear

A new size changes diameter, speedometer, and load on the wheel. Pressure, lug torque, tread depth, and a 12V inflator are the shop tools that keep the swap honest after the math.

AstroAI Digital Tire Pressure Gauge (0–150 PSI)

A size change does not set PSI for you. Door-sticker pressure on a plus-sized tire is still the starting point — confirm it cold, in 0.1 PSI, not by kicking the sidewall.

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EPAuto 1/2" Drive Click Torque Wrench (10–150 ft-lb)

Passenger-car lug nuts live in the middle of this range. After a size swap, torque to the door sticker or service manual — not “good and tight” with an impact.

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JACO ElitePro Analog Tire Gauge (100 PSI)

ANSI-calibrated analog, no batteries. Use it to cross-check a digital glovebox gauge after you fill a plus-size or a load-range change.

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AstroAI Digital Tire Tread Depth Gauge

AWD/4WD hates mixed rolling diameters. Measure remaining tread at the same spot on each corner before you mix a new size with a worn one.

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AstroAI 12V Portable Tire Inflator

Set cold pressure after the swap. Plus-sizing often needs a different inflation to carry the same load on a shorter sidewall — the door sticker is still the first number, then the load table.

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Tire size and speedometer error calculator

Run the Diameter Numbers

Original vs proposed size. Diameter %, speedometer error, revs per mile, and a ±3% fitment flag.

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How to Use This Tire Size & Speedometer Error Calculator

Type the size on the tire you have (or the door placard) and the size you are considering. The calculator builds overall diameter from width, aspect ratio, and rim, then reports how far the new circumference moves the speedometer, the odometer, and revolutions per mile. If the diameter change is outside ±3%, you get a clear flag — not because the math failed, but because that is the band most shops treat as “mount it and leave the cluster alone.”

Overall diameter (in) = rim + 2 × (width_mm × aspect / 100) / 25.4
Indicated mph = true mph × (D_original / D_new)

What each number on the sidewall means: size-code guide. ABS lights, fender rub, AWD binding: troubleshooting. If you are plus-sizing for looks vs matching diameter on purpose: plus-sizing vs same-diameter.

Tire Size & Speedometer Error Calculator