Why a taller tire lies to your speedometer
Your speedometer doesn't measure speed directly. It counts how fast the driveline is spinning and converts that to mph using the tire diameter it was programmed for. Put on a taller tire and every revolution now covers more ground — but the gauge still thinks you're on the old size. So the needle reads low: you're going faster than it says, and your odometer is quietly under-counting your miles.
How tire diameter is figured
A size like 285/75R17 isn't a diameter — it's width, sidewall ratio, and rim. To get overall diameter in inches:
Diameter = Rim + 2 × (Widthmm × Aspect% ÷ 25.4)
So 285/75R17 works out to 17 + 2 × (285 × 0.75 ÷ 25.4), or about 33.8 inches. The calculator does this for both tires and compares them, so you can see at a glance whether you're crossing that 3% line where a recalibration becomes worth doing.
Bigger tires change more than your speedo
- Gearing. A taller tire is like installing a numerically lower axle ratio — it drops your engine RPM at a given speed and softens acceleration. The calculator shows your new effective ratio and the axle ratio you'd need to get your original gearing back. Plan that on the gear ratio calculator.
- Ride height. Overall diameter change is split top and bottom, so your ride height rises by half the diameter change.
- Odometer and mileage. Under-counted miles throw off oil-change intervals and, for a work truck, your logged mileage.
- Fuel economy. Until you re-gear, a big tire jump usually costs you a little MPG from the taller effective gearing and heavier rotating mass.
