Speedometer Error Calculator

Most countries permit speedometers to read up to 10% high (pessimistic - showing faster than actual) but prohibit reading low (showing slower than actual). The EU standard requires speedometers to read between actual speed and actual speed + 10% + 4 km/h. Many factory speedometers read 2-5% fast by design. If your new tires make the speedometer read lower than stock, that may be illegal in some jurisdictions.

Yes. ABS and stability control use wheel-speed sensors that are calibrated to the OEM tire's revs-per-mile. A significantly different rolling diameter changes wheel-speed readings and can affect how these systems intervene. Very small changes (under 2%) are generally safe. Changes over 3-5% should be verified with the vehicle manufacturer or a qualified shop, particularly for AWD vehicles where mismatched rolling radii cause drivetrain stress.

Plus-sizing means moving to a larger-diameter wheel while using a lower-profile tire to maintain approximately the same overall rolling diameter. A properly executed plus-size (within 1-2% of original diameter) keeps speedometer and odometer error to a minimum while giving you the wider wheel and lower sidewall look. This calculator helps verify that the plus-size combo you are considering stays within the acceptable diameter range.

A taller tire has a larger circumference, so it takes fewer revolutions per mile. This slightly reduces engine RPM at a given speed, which can marginally improve highway fuel economy (0.5-2%) if the transmission and final drive ratios allow. However, a wider tire adds rolling resistance, which partially offsets the benefit. A tire that is 3-5% taller typically results in negligible net fuel economy change.