I don't think the car would run on farts; being largely methane, farts are LESS compressible than air.

Note (just by the way) that the energy required to fill a compressed air car with high-pressure air nd drive it, say, 100 miles, is probably a deal GREATER than the energy required to fill it with enough regular gas to drive that same 100 miles. The roads would be cleaner, but the net effect on atmosphere would not (unless the plants were solar, nuclear or wind powered).


A