Ooops.  Forgot to add.  Chuffers, at least the one I've got (GH Meiser & Co.) actually use the engine compression to cause a diaphragm to go up and down to pump air and does not blow an air/fuel mixture into the tire.  Others may vary.  It was handy on my IH Scout, FJ40 and Samurai, only 'cause the plugs are fairly accessible.  I'm not sure if it would fit well and I wouldn't want to cross thread a plug on my Vanagon.

Mike