How to measure speed without any mechanical or electrical connection?
A wireless bicycle speedometer offers the features you require.

Commercial vehicle test systems use a bicycle wheel bracketed to the rear of the vehicle.



Figure 1. PEGASEM 5th wheel for vehicle testing.

Figure 1 shows such a device.

The units are digital and as accurate as the wheel circumference data that you enter - typically 1 mm on a 2 m circumference (26" wheel).