LCAs have been done on the use of methane. Methane can be captured from watewater treatment plants and used to power that facility.

As for this particular source and use as engine fuel, why not do it while you can? People are quick to talk about engine performanc and blah blah blah, but think about how many vehicles are on the road where the most action they get is getting up to speed on an on-ramp or hitting the gas when the light turns green. Hybrid vehicles, fuel alternatives, gas mixtures have a reason for use because most vehicles are parked in traffic or in line at a drive thru. Fuel alternatives can be cleaner burning and reduce the waste of gasoline.