The only way I see Hydrogen being viable (and greener) is if you generate the Hydrogen using Solar or Wind power. Any other way you are just transferring the source of the pollution from one location (cars tail pipe) to another (coal, oil power plant). Basically the same thing is true for an electric car.

Years ago (early 1980's) guys were installing water injection systems in their cars and reporting a 10% - 20% increase in gas mileage. When I lived in Anaheim, CA a couple of guys I worked with did this. Another co-worker had a early 60's Mustang that would run on Gasoline or Propane. He keep two 20 pound Propane tanks in the truck. He had a switch under the dash for switching from gas to propane (he could only switch with the engine off). It was a very simple system that allowed him to use either type of fuel.

Tom

Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety. Benjamin Franklin, US author, diplomat, inventor, physicist, politician, & printer (1706 - 1790)