Why? Its not that easy to use as a vehicle fuel. There have been tax incentives before, but they never took off. The local gas company in my area opened maybe a dozen or so fueling stations around the state. After a couple of years they closed them. Did you buy a NG vehicle? Too bad for you.
These tax credits are way subject to gaming. Arizona a few years ago offered a substantial tax credit, like 25% for alternative fuel vehicles. A couple of glitches. For one thing, the state income tax wasn't all that high. To really take advantage of it you had to be a high-income person. Then, there were no standards. So conversion companies put in the natural gas regulator thingy in the engine compartment (cheap) and then put in a token tank (the expensive part of a natural gas vehicle). It almost bankrupted the state.
I'm a lefty, but I'd get rid of all these subsidies and start over. Principle number one: Cost causers should be cost payers. So gas tax nationwide should rise 50 cents per gallon. That would cover costs, with a nickel to catch up on the backlog of road repair and another nickel for natural gas subsidies if we want.
Natural gas looks better and better. There would be about that extra nickel for subsidies within that 50 cents that would mildly discourage unnecessary driving of low mileage vehicles. No need to be draconian. CAFE could be abolished since the HumVee owners would be disproportionately contributing to the solution. The nickel could be used to pay for natural gas subsidies if we want or electric car subsidies.
Right now we are subsidizing both gas and alternative fuel vehicles, and that's just dumb.