Biodiesel has less BTU's then Low sulphur diesel(LSD). However Ultra low sulphur diesel(ULSD) has less BTU's also. ULSD will also degrade natural rubber fuel lines. In North America all OEM's switched to synthetic lines in 1993. As far as fuel economy some users are getting better mileage from low level blends(like B20) due to higher Cetane, higher Lubricity , and Biodiesel is 11% Oxygen by volume. These three properties combine to produce a more complete burn of the petroldiesel portion of the fuel.

Gary T