They probably use GPS? If not, then just get bigger tires.

Speedometers measure speed by counting wheel rotations and multiplying them by the circumference. There's at least a few percent inherent error as circumference is going to vary with what tyre is fitted and how worn it is.

F1 cars, and I believe now high end sports cars use a differebt system that's basically how an optical mouse works. They image the road underneath the car and measure it moving past. This gives turning in addition. If you watch an F1 night race you can see a glow on the ground under the nose - that's the illuminator for the sensor.

In some jurisdictions the law says that the car speedometer must show a higher speed than the correct one (as long as tyres aren't above the correct pressure), so that the driver can "trust" the speedometer to avoid them a speeding ticket. On my car 140 km/h on the speedometer corresponds to 128-130 km/h on the GPS.