IMHO rather than busting the big 3's ( or 7 depending upon which auto analyst is describing the industry ) cohones about emissions, I think greater gains in clean air can be had with industrial engines ( off road ) such as a pump, a driveway tamper when they are prepping for concrete or asphalt or even a weedwacker by redesigning them to run on Propane or Natural Gas. EFI and new tankage systems would have to be developed, but the cost of the fuel with the fracking revolution and the cleaner air might make it a win win for the contractors ( and individuals ) that buy these devices.
You are in fact correct. What exists is a trend.
We’ll see if the slope is positive or negative.