Save the environment and go straight HO2...forget about gas/electric. The govt did it ages ago, many people are doing it...it's just a hush-hush or you might get shot...funny but true in some cases. gas/electric hybrids will give you anywhere from 40-60mpg depending on the car, ect. HO2/gas will do probably an extra 15-25mpg, but your car will last alot longer and run much cleaner and you will gain a few extra horses. The cost for this transformation would run about 2-4k for parts/labor depending on vehicle model, ect. If you can read an owners manual, you can install it yourself or the average mechanic can also do it. Since it's only vaccum lines running across your engine bay. H02 will give the best bang for the buck in the long run. Up to 100mpg can be achieved by pure HO2 no gasoline here. Major increase in horsepower/tq. Cost will depend on parts being new/used and also labor anywhere from 4-10k for parts/labor/tuning depending on the vehicle model, ect. Hydrogen is the cleanest burning fuel of all time. It will reduce carbon deposits. The result is a longer lasting and more efficient engine. But anyways continue with the conventional hybrid talk.