Calculate how much your speedometer and odometer over-read or under-read after switching to a different tire diameter, and find your true speed and true distance.
📂 Automotive & TransportationFill in the fields on the left, then press the button to see your result instantly. Everything runs locally in your browser — no sign-up required, and no data is ever sent anywhere.
Your car's speedometer and odometer don't measure road speed and distance directly — they count how many times your wheels rotate and convert that count into a speed or distance reading using a calibration factor set by the manufacturer for your vehicle's original tire size. That calibration assumes a specific tire diameter, because a tire's overall diameter determines exactly how far your car travels for each full wheel rotation (its rolling circumference). If you install tires with a different overall diameter than stock — whether intentionally for looks, off-road capability, or accidentally through a tire size that isn't quite equivalent — every wheel rotation now covers a different real-world distance than the calibration assumes, so both your speedometer and odometer become systematically wrong by the same percentage. This calculator uses the ratio between your new and original tire diameters to work out exactly how far off your readings are, showing you your true speed for any indicated speedometer reading, and the true distance behind any odometer or trip counter value.
A speedometer was never really built to measure speed in the way a radar gun does — instead, it counts wheel rotations (or, on modern cars, pulses from a wheel-speed or transmission output sensor) over time and converts that count into a speed reading using a calibration factor baked in at the factory. That calibration factor is derived directly from the rolling circumference of the tires the vehicle originally shipped with, which is why the moment you install a tire of a meaningfully different overall diameter, the underlying math the speedometer relies on is no longer accurate for your car.
The relationship is a simple ratio: divide your new tire's overall diameter by your original tire's overall diameter to get a scaling factor, then multiply your indicated speed by that factor to get your true speed. A larger new diameter means each wheel rotation now covers more ground than the speedometer assumes, so your true speed is higher than what's displayed — the speedometer under-reads. A smaller new diameter means the opposite: each rotation covers less ground, so your true speed is lower than displayed and the speedometer over-reads. The exact same ratio applies to the odometer and trip counter, since they're driven by the same rotation count.
This shows up constantly in two very different real-world scenarios. Off-road and overlanding enthusiasts often fit noticeably taller, more aggressive tires for ground clearance and traction, which can introduce a speedometer error of five to ten percent or more if left uncorrected. On the other end, some performance "plus sizing" setups — swapping to a larger wheel with a lower-profile tire — are specifically chosen to keep the overall diameter close to stock precisely to avoid this kind of speedometer and odometer drift.
The practical consequences go beyond a mildly wrong number on the dashboard. If your speedometer under-reads because of larger tires, you could be driving faster than you realize and risk a speeding ticket while genuinely believing you were within the limit. If your odometer under-reads, your logged mileage will fall behind your actual mileage, which can throw off maintenance intervals (like oil changes scheduled by mileage) and even affect resale value calculations or warranty mileage tracking, since the odometer will show fewer miles than the vehicle has truly traveled.
The good news is that many modern vehicles — particularly ones with a fully electronic instrument cluster — allow a speedometer recalibration through the onboard computer, a dealer service visit, or an aftermarket tuning device, letting you correct the readings to match your new tire size exactly. Until that's done, or if your vehicle doesn't support recalibration, knowing the precise correction factor (as this calculator provides) lets you mentally adjust your speedometer and odometer readings so you always know your true speed and true distance traveled.
Your speedometer doesn't measure road speed directly — it counts wheel rotations per unit time and converts that count to a speed using a calibration factor based on your original tire's rolling circumference. Installing a tire with a different overall diameter changes how far the car actually travels per wheel rotation, so the same rotation count now corresponds to a different real-world speed, throwing the calibration off.
Larger-diameter tires cover more ground per rotation, so at any given wheel rotation rate your car is actually traveling faster than the speedometer — calibrated for the smaller original tire — indicates, meaning larger tires make your speedometer under-read (show a lower speed than you're really going). Smaller-diameter tires do the opposite and make it over-read.
Yes — the odometer and trip counter use the exact same wheel-rotation-based calibration as the speedometer, so the same percentage error applies to distance readings: larger tires make your odometer under-count the distance you've actually traveled, while smaller tires make it over-count.
On many modern vehicles, yes — some allow a speedometer recalibration through the onboard computer or a specialty tuning device, and some aftermarket gauge shops offer physical recalibration services; check with your vehicle manufacturer or a qualified mechanic, since availability varies by make and model.
Yes, indirectly — because tire diameter changes the effective overall gear ratio between the engine and the road, taller tires act like a slightly higher (numerically lower) gear ratio, generally increasing potential top speed but decreasing acceleration and low-end torque at the wheels, while shorter tires do the opposite.