I think the point was nailing down when it raises and lowers on the speedo.
The whole "4-5mph over" thing doesn't necessarily apply to all 986's.
Mine is 2mph over. My spoiler raises at 75 on the speedo (as all 986's do), which is 73 actual.
In other words, the actual speed is irrelevant as the speedo readout difference will vary slightly from car to car.
They all always go up at 75 on the digital speedometer.
