308 GT4 rev counter accuracy

I've recently fitted 2 x MSD units to the GT4 together with the MSD tach adapter and an MSD rotary rev limiter. I've set the MSD limiter at 7,000 rpm but noticed the revs on the rev counter showing 8,000 +. More like 8,200. Obviously one of the units is not right. I'm inclined to think its the GT4 rev counter but is there any way of checking the actual revs of the engine?