Our two favourite cynics, Jamie Hyneman and Adam Savage from the Discovery channel's Mythbusters, recently turned their (sometimes uncomfortable) spotlight on the bikers' long-held claim that motorcycles are way greener than cars.
And, in true Mythbusters style, they discovered that in some ways it's true - and in other ways it ain't.
Let's elucidate: The team enlisted the help of Dr Kent Johnson from the University of California Riverside and his super-accurate gas-sniffing equipment, and tested a cross-section of cars and bikes from new to 30 years old over the same real-world driving loop, 75 percent freeway and 25 percent city traffic.
The first thing they confirmed was that the bikes used less petrol than the cars to travel the same distance. That's no surprise; you don't need sophisticated equipment to prove that, just a log-book.
They also showed that the bikes produced less carbon dioxide than the cars - once again, that's no surprise; 90 percent of what comes out of the tailpipe of any petrol-powered vehicle is CO2, so if you use less petrol, you'll produce less CO2.
But it's in the other 10 percent of tailpipe emissions that the big surprises were lurking.
Turns out bikes produce more unburnt hydrocarbons (which are carcinogenic), more nitrogen oxides, which cause acid rain, and as much as 8000 percent (80 times!) more carbon monoxide (an odourless, lethally poisonous gas) than cars.
A controlled test using a modern, small-capacity, fuel-injected bike under ideal conditions on a racing circuit didn't help much, and neither did enclosing the bike in a aerodynamic frame to cut wind resistance.
The fundamental problem seems to be that motorcycles are performance vehicles, built to wring every last watt out of every last revolution of their engines, whereas cars are built to run as clean as possible because they have to be, by law.
Sure, a 600cc sports bike produces more power than a 1.4 hatchback but, even though the car uses more fuel than the bike to drag around its many-times greater weight, it actually uses that fuel more efficiently, and cleans up after itself better than the two-wheeler, thanks to a bigger, more efficient catalyser.
Theoretically, it's possible to build a motorcycle that would run as clean as a car, by tuning it to run super-lean and fitting a big, ugly catalyser, but the resulting scoot would be slow, uncomfortable and not much fun to ride.