Tire Size Speedometer Error Calculator

Taller tires: the wheel turns fewer times per mile, the speedometer counts fewer pulses, and it under-reads. A 4% taller tire at an indicated 65 mph means a true 67.6 — enough to flip a borderline radar reading. Factory speedometers are deliberately biased to read slightly high (UNECE rules forbid reading low at all), so a modest tire increase sometimes just cancels the factory optimism; a big lift-kit tire swap blows straight past it.

It's proportional, not constant: a 3% diameter increase under-reads 3% everywhere — about 1 mph at 30 and 2 mph at 70. That's why correction is a single multiplier, and why a recalibrated speedo (dealer reflash, programmer, or corrected reluctor settings) fixes all speeds at once rather than needing a lookup table.

Same ratio, same direction: taller tires mean the odometer logs FEWER miles than you actually drove (100 true miles records as ~96 on 4% taller rubber). Fuel economy then looks worse than reality, service intervals stretch in real-world miles, and a high-mileage truck on big tires has quietly done more miles than the title says. Smaller tires inflate recorded mileage — the direction lease companies care about.

Three routes: a dealer or handheld programmer reflash that stores the new tire revolutions-per-mile (cleanest, most modern trucks), a driveline speed-sensor gear change on older cable/gear-driven setups, or an inline pulse calibrator on electronic senders. GPS speed from a phone app is the honest referee for verifying the result — average both directions on a flat run to cancel wind and grade.