Only a lawyer would try such rationale
The only way the state government can draw the line on who does/doesn't get such a benefit is by the bottom line. The delta change in improvement is too difficult for the state to process/enforce. And a delta change logic leads to such sillyness like giving congradulations to someone for giving up a clunker for a V-8
Plenty of folks who work on mileage issues for a living who think gallons per mile (or liters per 100 km) is inherrently better than the way we regulate. Economists more than lawyers, but policy folks in general. Read up a bit. You've got the time down in Duck.
Sure, the ideal conservation case for would be to put everyone in Leafs with batteries powered by solar arrays and wind turbines, but that isn't happening without a major change in consumer, voter, and political preferences, and would have safety and employment implications. In terms of reduction in gasoline demand, at least in the near term, it is more important to improve the least efficient vehicles in your fleet by a larger percent on an MPG basis than it is to improve more efficient cars by more MPG. Get folks to move to high-rises in Arlington and take mass transit is great, but when you are dealing with development that has taken place in Loudoun and Stafford, with 2 car families that commute in different directions, and at least one vehicle has to be able to sit 2 kids and maybe the baggage like a stroller, getting that vehicle more efficient will save more gas than getting the other car changed from an Accord to a Prius.
As for enforcement, what gets measured gets managed. At least with new cars bought from dealers, this is easy to enforce, in comparison to other standards. Most of the measures that are taken now to rate cars efficiency at the manufacturer and ensure proper operation on the road can be applied to gallons per mile format.