How New Tire Sizes Throw Off Your Speedometer and Odometer

Your car doesn’t actually know how fast it’s going. It counts how many times the wheels turn and multiplies by an assumption — the rolling circumference of the tires the factory fitted. Change that tire size and the assumption is wrong, so every number the car reports is now off by a fixed percentage: the speedometer, the odometer, the trip meter, and the fuel-economy readout all drift together. Fit a plus-size wheel, a chunkier all-terrain, or even just a worn tire, and you’re quietly driving faster (or slower) than the dash claims. This guide explains why, and how to work out the real figure.

The car measures revolutions, not distance

Speed is distance over time, but a car has no odometer laser measuring the road. It measures wheel rotation — from an ABS sensor or the transmission output — and converts revolutions to distance by assuming each turn covers one tire circumference. The factory calibration bakes in the original tire’s circumference. When the tire changes, the conversion factor is stale, and the error propagates to everything downstream.

The physics is a single ratio. A tire that is taller (larger overall diameter) covers more ground per revolution, so at any given rotation rate the car is actually travelling faster than it thinks. The speedometer, still assuming the small original circumference, reads low — it under-reports your speed. A shorter tire covers less ground per revolution, so the speedometer reads high. That’s the whole mechanism.

The math: it’s just a diameter ratio

The correction is beautifully simple. Your true speed is:

true speed = indicated speed × (new diameter / old diameter)

Everything hinges on those two diameters, and both come straight off the sidewall — a 225/45R17 works out to an overall diameter of 634.3 mm. If you want the decode and the arithmetic behind that figure, the guide to reading the sidewall and plus sizing walks through it; here we only need the finished numbers.

Suppose you move to 225/50R17 — same width, same wheel, but a taller 50% sidewall, which brings the overall diameter to 656.8 mm, about 3.5% taller. Now when the dash reads 100 km/h, you’re really doing 100 × (656.8 / 634.3) ≈ 103.5 km/h. You’d be speeding by 3.5 km/h while believing you were exactly on the limit. Go the other way to a shorter tire and the speedo would over-read, so you’d unknowingly drive slower than the dash shows.

The Tire Size Comparison calculator does this for you: enter your original size and the new one and it reports the overall-diameter change as both millimetres and a percentage, then takes an indicated speed and shows your true speed. It also lists revolutions per mile and per kilometre for each tire — the number the car’s electronics actually care about.

Why the odometer and fuel economy drift too

The speedometer error isn’t a standalone quirk; it’s the same ratio applied to distance. Because the odometer counts the same revolutions, taller tires make it under-count distance — you’ve genuinely driven farther than it records. Over a year that adds up:

• Odometer / mileage. With 3.5% taller tires, a real 10,000 km shows as about 9,660 km on the dash. Your car looks like it has fewer miles than it does — which matters for service intervals, warranty, and resale honesty.
• Fuel economy. MPG and L/100km are computed from that same understated distance. Taller tires make the trip computer report worse economy than reality (it thinks you went less far on the fuel you burned); shorter tires flatter it. If your mileage figures jumped the day you changed tires, the tires — not the engine — are the reason.
• Trip timing and cruise control. Cruise holds an indicated speed, so with taller tires it’ll actually hold you a few percent faster than the set number.

How much change is “too much”?

A small deviation is normal — manufacturers allow a tolerance, and speedometers are legally permitted to read slightly high (never dangerously low) in most regions, so a modest taller tire can actually bring an optimistic speedo closer to truth. As a rule of thumb, aim to keep overall diameter within about ±3% of stock when changing sizes — beyond that the error stops being academic, and clearance and electronics start objecting too. The plus-sizing and fitment guide covers what else that ±3% window is protecting.

If you’re deliberately going bigger — say lifting a truck onto larger all-terrains — you’re not just changing the speedo reading; you’re effectively changing the final drive ratio, because the engine now turns fewer times per mile. That affects acceleration, towing, and where the engine sits in its rev range at cruise. When the change is large enough to matter, people often re-gear the differential to compensate. The Gear Ratio & Tire Size calculator shows how a new tire diameter shifts your effective gearing and what axle ratio restores the original feel.

Don’t forget tire wear and pressure

Even without changing sizes, your rolling circumference isn’t constant. A brand-new tire and a worn-to-the-bars version of the same size differ by several millimetres of tread depth — enough to nudge the speedo a fraction of a percent. Under-inflation squashes the tire and shrinks the effective radius slightly too. These effects are small next to a deliberate size change, but they’re the reason two identical cars can show marginally different odometer readings over their lives.

Check before you fit

The appeal of a plus-size or a beefier tire is real — better looks, more grip, more ground clearance — but it quietly rewrites every speed and distance number your car shows you. The good news is that the correction is a single, predictable ratio, so there’s no reason to guess. Before you buy, drop both sizes into the Tire Size Comparison calculator, check that the overall-diameter change stays within a few percent, and note your true speed at an indicated 100. If you’re making a bigger jump, run it through the Gear Ratio & Tire Size calculator too. Know the number, and you can either choose a size that keeps your speedo honest — or at least drive knowing exactly how much the dash is fibbing.