Spedo/Emissions test on 2-wheel dynamometer

["Chuck Schott" <chucks@perteet.com>]

A local speedometer  shop jacked the rear-end of my 4kq and had to lock the
center differential to test my speedometer with the front wheels on rollers.

Is this method of testing damaging to the drivetrain?

Using the roadside speedometer check signs (timing over a distance of 5
miles with speedo reading 60 mph), my speedometer reads approx. 3mph more
than actual speed.  By shop-dyno test my speedometer reads 1mph less than
actual speed. (both at 60mph)

Why the difference?  Which is more accurate?
Does the speedometer on the 4kq correlate with the front wheel speeds, rear
wheels, or does the Quattro system allow different front and rear wheel
speeds (even with center differential locked) which would make the front or
rear wheel speed inaccurate?

Thanks in advance.
Chuck
Seattle, WA