How to Calculate Speedometer Error After a Tire Size Change

Actual speed equals indicated speed multiplied by the new tire's overall diameter divided by the old one: actual = indicated x (D_new / D_old). The speedometer counts wheel rotations and reports speed based on the diameter the factory tuned it for; fit a taller or shorter tire and every rotation covers a different distance, so the reading is off by the same ratio at any speed.

A speedometer does not measure road speed directly. It counts revolutions of the driveshaft or wheel speed sensor and converts that count to mph using the diameter (and final drive ratio) the vehicle shipped with. A larger-diameter tire covers more ground per revolution than the sensor's math assumes, so the dash under-reports actual speed. A smaller-diameter tire does the opposite: it over-reports. The ratio is constant across the speed range, which is why a single diameter comparison predicts the gap at 30 mph as well as at 70 mph.

Running 225/45R17 and 235/45R17 through the geometry engine gives an overall diameter of 24.97 in (634.3 mm) for the original size and 25.33 in (643.3 mm) for the replacement, a difference of +0.35 in (+9.0 mm) (+1.42%). Applying actual = indicated x (643.3 / 634.3) to three common dash readings:

At every reading the car is moving about 1.4% faster than the dash shows, because the new tire is 1.42% taller and covers more distance per rotation.

The formula only needs the two overall diameters, which come from width, aspect ratio and rim diameter -- run both sizes through the tire size calculator to get them, then divide new by old and multiply by whatever indicated speed matters (a highway cruise speed, a school-zone limit, a cited speed on a ticket). A GPS speed app or a highway mile marker gives an independent check against the calculated ratio. Some vehicles let a dealer or a specialty tuner reprogram the speedometer's reference diameter after a permanent size change; that is a separate step from the math here, and whether it is worth doing depends on how large the ratio turns out to be.

Mechanical, cable-driven speedometers and modern electronic ones that read a wheel-speed or transmission output sensor are affected the same way, because both are ultimately counting rotations and converting the count using a diameter figure baked in at the factory. Cruise control set to an indicated speed will also hold the same wrong actual speed as the speedometer -- it uses the same sensor input, not a separate GPS-based measurement. Traction control and stability systems that compare wheel speeds to each other are less affected, since an even change across all four tires keeps the relative comparison intact; a size change on only two tires (front or rear) is a different, larger problem outside the scope of this formula.

Most vehicles built since roughly 2010 read speed from a wheel-speed sensor or the transmission output shaft and convert that signal to a dash reading in software, not with a physical gear. Per AutoZone's recalibration guide, the fix on these is "plug the scanner or tuning device into your vehicle's OBD-II port" and then "enter the updated tire size, gear ratio, or other relevant information to adjust the speedometer reading" -- a handheld tuner, a dealer scan tool, or in some cases a menu built into the dash itself. AutoZone also notes plainly that "there are a great number of vehicles that require a visit to the dealership or another repair shop to calibrate the speedo" -- this is not a universal owner-DIY procedure, and which path applies depends on the make and model.

On the older mechanical design -- a cable turning a speedometer head through a drive gear and a driven gear -- the fix is a physical part swap, not software. AutoZone describes it as "physically changing the gear or the cable with one that has either more teeth -- to make your speedo read slower -- or less teeth -- to make your speedo read faster." The driven gear's tooth count is sized with the formula from Inch Calculator's speedometer gear calculator: driven teeth = (drive teeth x axle ratio x revolutions per mile) / 1,001. This applies to older vehicles and dedicated off-road/swap builds still using a cable speedometer; it does not apply to a modern vehicle with an electronic speed sensor, which has no driven gear to swap.

As a rough guide to when it is worth doing either fix: an error under about 3% is rarely noticeable in daily driving and many owners leave it as-is; once the gap passes that range -- as it will for most plus-size changes of more than one wheel diameter step -- checking the dash menu or an OBD tool for a tire-size setting is worth the few minutes it takes.

Because the error is a fixed ratio, it applies at any indicated speed, including a posted limit or a speed cited on a citation. Multiplying the posted number by the same D_new/D_old ratio gives the actual speed the vehicle needs to be doing for the dash to read that limit. For the 225/45R17 to 235/45R17 example above, a 65 mph indicated speed on the new tires corresponds to about 65.9 mph actual -- the same 1.42% gap as the 30/50/70 mph rows in the table.

Discount Tire's own calculator disclaimer states that "the calculator results are based on the mathematical equations of the sizes entered, not the specifications provided by the tire manufacturer" -- the same principle applies here: this page's numbers come from the two sizes' geometry, not from a manufacturer-measured rolling diameter or a specific vehicle's speedometer calibration. Full limits are covered on the site disclaimer page.