How a different tire size changes your speedometer
Your speedometer does not directly measure speed. It measures wheel rotation and converts that to a speed reading based on one assumption: the rolling circumference of the tire. Change the tire size, and every speed and distance reading changes proportionally.
How the speedometer works
A tone wheel on the axle passes magnetic sensors, generating pulses per revolution. The ECU divides those pulses by the tire's expected revolutions-per-mile to derive speed. That expected RPM is fixed at the factory for the OEM tire size.
The speed formula
actual_speed = indicated_speed × (OD_new / OD_stock)Where OD = overall diameter in any consistent unit (mm, inches — it cancels out).
Worked example
Stock size: 215/55R17 — OD = 668.3 mm
Replacement: 225/45R17 — OD = 645.3 mm
actual_speed = 60 mph × (645.3 / 668.3) = 57.9 mphWhen your speedometer reads 60 mph, you're actually going 57.9 mph — the speedo reads high because the smaller tire rotates more times per mile than expected.
The TreadSize compare tool computes this instantly for any pair of sizes.
The sign convention
| New tire vs. stock | Speedo reads | Odometer reads |
|---|---|---|
| Larger diameter | Low (shows less than actual speed) | Low (under-counts miles) |
| Smaller diameter | High (shows more than actual speed) | High (over-counts miles) |
Odometer accuracy
The odometer error is the negative of the speedometer error — they move in opposite directions:
odo_error_pct = -speedo_error_pctA larger tire: speedo reads low (you go faster than it shows), odometer records fewer miles than you actually drive. A smaller tire: the reverse.
Over 100,000 miles, even a 2% diameter difference means the odometer is off by 2,000 miles — relevant for warranties, lease agreements, and resale value.
ABS and traction control
ABS wheel speed sensors work on the same rotation-count principle. When all four tires are the same size, the sensor readings are consistent and the ECU detects slip correctly. If you install significantly different-diameter tires (front vs. rear, or all-around non-OEM), the system may interpret the diameter difference as wheel slip and activate ABS or stability control falsely.
This is why the 3% rule is the practical limit for most street-driven vehicles without a speedometer recalibration.
Legal considerations
In most US states, speedometers are required to read at or above actual speed (not below). A tire that makes your speedo read low means you could be traveling faster than indicated — a potential traffic enforcement issue. The permissible error varies by jurisdiction but is typically ±4% or ±4 mph, whichever is greater.
When does it matter most?
- Speed-limit compliance — if your speedometer reads low, you may be driving faster than you realize.
- Lease mileage — odometer overcounting (smaller tires) can put you over your mileage limit; undercounting (larger tires) may hide actual wear.
- Warranty claims — non-OEM tire sizes may void or complicate powertrain warranty claims.
Calculate your speedometer error
Use the TreadSize compare tool — enter your stock size and the replacement size, and it shows exactly how fast you'll be going when the speedometer reads 60 mph, the percentage error, and the corresponding odometer drift.
Speedometer formulas are based on standard rolling circumference math. Laws vary by jurisdiction. Consult a qualified tire professional before changing tire sizes, especially if you have ABS, ESC, or TPMS systems.