As part of the second Boston GreenFest, seven teams will attempt to drive their homemade cars from Greenfield to Boston City Hall Plaza, a distance of 100 miles, using just 1 gallon of gasoline. Event organizer Jory Squibb, who in 2005 fused two Honda motor scooters to create a fuel-efficient microcar called "Moonbeam,'' said the message he hopes the One Gallon Challenge sends is that 100-mile-per-gallon cars are not impossible (or costly or difficult) to build. The technology is there, he said; all that's needed "are gutsy entrepreneurs and gutsy buyers.''