Digital speedometers use computer pulses from the sender at the transmission output to
drive the rate counter in the speedometer unit itself which in turn controls the speedometer
needle or display. Neoplan no doubt buys that unit from someone else, and it is probably a
"universal" controller that can be fitted to a metric Km/h or English/US readout in MPH.
The speedometer probably has a DIP switch (DIP means dual-inline package) that must be
set to match the pulses per tire/driveshaft/whatever they use coming from the sender.
The new speedometer is probably compatible with the sender as well, but you may have
to set the switches to get the correct speed readout.
I'd set the speedometer in place, get the bus out on a straight stretch of highway that
has mile markers, put it up to 60 mph (or 45 or 30) and time it. 60 mph = 1 mile/minute;
45 = 1 mile/1.5 minutes, 30 = 1 mile/2 minutes, etc. Then check the odometer against
mile posts on an interstate highway over a distance of 5 or 10 miles to verify accuracy.
It's not rocket science, but it's worth the trouble to know your speedometer and odometer
are accurate.
Clarke