As long as your car came from the factory with the 20" wheels, the PCM's tune will calculate speed using their diameter. It also expects the OE tire size to be on those rims, but it would take a significant change in tire size to equal 5mph difference.
It's much more likely that the roadside radar is incorrect. The majority of those I have seen were usually 3-5 mph off (most use K band radar, which could contribute to the discrepancy).
Also, the angle at which your car is from the radar has an effect - approximately 1 mph difference for every 10 degrees off to the side you are if I remember correctly.