For what it is worth...
Couple of things to consider (if you haven't already). If the tires are the same size/type (extremely) identical, it is very easy to build one of these devices. Consider that the two wheels you are measuring will change size throughout the run, if the tires are different materials, widths or thicknesses, they will grow at different rates.
In some examples on streamliners, you will have turned aluminum wheels on the front and "traditional" rubber wheels on the drives. The drive wheels will grow at a faster rate and more by percentage than the aluminum wheels will. The different rate and amount of growth will show as wheel slip.
At lower velocities, the rate of growth will be less of an issue than at higher velocities. When designing for the faster velocities, you may need a more sophisticated logic circuit with some tire growth data to be used for making adjustments.
In the extreme example, the 28.5" MT Bonneville tire will grow 15% over static height at 400 mph while an aluminum wheel will grow at a much lower rate. If you do not compensate for the growth rate of the two tires, the faster you go (even with zero slip), your indication will be that wheel slip is occurring.
Even with two traditional wheels, running a Goodyear on the front and a MT on the rear, they will more than likely grow at different rates. It will not be as extreme as the turned aluminum/MT combination, but it will be different. Since you want to keep wheel slip to a small percentage (3-5%), the minor differences in growth rate might wreak havoc on your indications without applying corrections.