Sean,

 My '74 Westy <with aerodynamic handicap pop-top> has done 77 mph on
 flat ground with no apparent wind. My owners manual says that a '74 bus
 has a "top and cruising speed" of 79 mph. 

 I've had my bus well over 80 mph coasting downhill in neutral in pursuit of
 Bradley and his '77 on a steep descent enroute to the Panamint campout.

 Dave