I have a completely clear conscience about running my two Jawas tbh. With modern fully synth 2T, they are practically smoke free, and in some respects, NOx emissions particularly, they're much better than any 4T. If you also factor in the environmental consequences of manufacturing a car against a smallish bike, then I think its a no contest, particularly as the bike us likely to last at least twice as long as a car before it needs to be re-manufactured.