That is the least of it. The companion study on the Insight makes the following very-hard-to-believe comment that a heavier Insight gets better mpg.
nathan
"• UR-1 and UR-3 – The only difference is the maximum payload
added
during the UR-3 test. One would anticipate that the UR-3 result would
be lower, but it was actually 9.5 mpg higher. Normally, the heavier
payload (UR-3) would suggest that fuel economy would be lower,
especially during the many acceleration phases encountered during
urban driving. Not having access to the operating and control
algorithm for the Insight's motors and controller, one can only
guess
that the electrical motor may be utilized more with the heavier
payload. Therefore, the higher efficiency of electric motors may have
contributed to the results. The Honda literature does state that the
electric motor assists the gasoline motor when accelerating
"hard"."