How speedometer readings can deviate and why
Speedometers routinely display numbers that aren’t perfectly in line with actual vehicle speed. Engineers and researchers point out that intentional calibration allowances exist. A notable expert explained on radio that some margin is built into the system to account for real-world variation. (Source: Sputnik) The result is that the displayed speed often sits higher than the true speed by a small but measurable amount.
The typical range of this overstatement is about 5 to 10 percent. If vehicles showed speed inaccurately and owners were fined for speeding, while the instrument did not reflect the true pace, manufacturers could face legal actions. To reduce that risk, the design often leans toward overreporting speed rather than underreporting it. (Source: Sputnik)
Beyond the baseline calibration, tire and wheel factors can shift speed readings. The accuracy of a speedometer depends on the tire diameter and how much tread is worn. When tires are new and inflated to the maximum size allowed by the vehicle, the speed reading aligns more closely with real speed. Over time, as tires wear down and contact patch changes, the speedometer can drift—often by a small percentage—causing readings to be even less accurate. (Source: Sputnik)
Smaller wheel diameters force the drivetrain to rotate more to cover the same distance, which can push the speedometer to show a higher value. Many speedometers actually track engine or transmission outputs rather than speed directly, using rotational measurements from components like the output shaft or a wheel sensor to infer speed. (Source: Sputnik)
Conversely, installing larger-diameter wheels can lead to a speedometer underestimation. This discrepancy is a known consideration for drivers who change wheel size and rely on the instrument panel. In such cases, drivers may turn to independent devices, including satellite-based or GNSS-based systems, to cross-check speed. However, even these tools have limitations in accuracy and should be used with caution rather than as the sole source of speed information. Some riders and drivers opt for dedicated, high-precision speed measurement instruments for critical timing or navigation needs. (Source: Sputnik)
In practice, understanding how these factors interact helps drivers in North America interpret speed readings more reliably. It also underscores why regular checks with trusted measuring devices can be wise for those who depend on precise speed data for safety, fuel efficiency, or performance monitoring. (Source: Sputnik)
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