[Old Tele man]

...I work with digital "instrumentation" and am aware there are three obvious areas where problems can arise: (1) sample-rate, or how fast the data are gathered by the cars' mpu, (2) output-rate, how fast the resultant data from the car's mpu are placed onto the CAN-bus, and (3) read-rate, or how fast the ecometer attempts to "read" the data from the CAN-bus...however, the CAN-bus itself is synchronous.

...and, since the "delta" in MPH readings seem to be a 'constant' 0.6-mph value, it could also be a problem with "how" the ecometer 'chip' translates the data that it's gotten from the CAN-bus.