Assuming you wre only vehicle in the area when you passed the police warning device, I'd r'd think it was more likely that your dad's speedo was off rather than the radar device.

If you have a speedometer shop in your area, it's a fairly quick and easy job to check out the accuracy of your speedo, and they can re-gear it. If not, see if there are measured mile markers in your area and drive by them and check your odometer. Don't just check one mile -- drive thru about 5 miles and see how many miles your odometer has changed during the 5-mile run.

I'd also check the gearing on the differential. Check to see how many times the driveshaft turns during one wheel revolution.