Hi Rob,
I’d suggest to recalibrate your radar gun before the next flight.
Even if your K&B 7.5 fan engine unloads to 26.000 rpms in the air the theoretical speed (in a 'perfect' environment without any parasitic drag from an airframe etc.) produced by an 8" pitched prop is below 200 mph.
Maybe you want to measure speed using the Doppler effect for instance?