If power generation is not in the vehicle, our national fraction of combustion generators can be reduced systematically. This is already under way; some US cities are already at 0.
That leaves storage on vehicle. The more EV's we have, the more incentive to find clean, high energy density batteries. This is not a static field either. Graphene is just one vaporware which might appear on the street shortly.
However, my understanding of the overall picture is that car engines (both diesel and gas) are so much less efficient than a power plant, that even with transmission, it's cleaner to switch to centralized power generation.
For gasoline the number is supper tricky to determine. So here comes a back of napkin number
A fuel truck transports about 30,000 gallons (20,000-40,000). On average they achieve about 4-7.5 miles per gallon). So they use about 1% of their tankage for every 60 miles traveled.
This suggest to me that transportation loses for gasoline are in the same ball park number. Distribution and marketing is often claimed to be about 5% of the total, but I don't know where that number comes from.
All in all the difference is probably not that large, and I suspect the electricity one will drop faster than the gasoline one due to interconnect upgrades.
Battery cost is still dropping at an exponential rate while energy density continues to rise. Likewise solar panel costs, and installation (especially at utility scale) is also coming down in price as the industry learns.
I’m no expert, but I predict that within 10 years it will be entirely practical (and much more environmentally friendly than diesel/gasoline, especially w/r/t urban smog) to have bus fleets, delivery trucks, etc. running on batteries.