55mph is stored at 89kph in the Waze data. The math done by the script ends up rounding to 56 instead of 55. The following code change works better I think:
fwdspeed = (fwdspeed!=null) ? (Math.trunc(fwdspeed0.625)) : null;
revspeed = (revspeed!=null) ? (Math.trunc(revspeed0.625)) : null;
RANT: not sure why Waze uses .625 is, as the conversion when .621 is closer. And 0.6213711922374 is significantly closer to the actual value.