>>10220678mpg isn't a useful measure, because it doesn't incorporate work done. Emissions correspond to work done, as long as each vehicle subset is the same. I can confidently assume they're not (how many electric trucks do you see?) but speaking physically, electricity is getting cheaper to generate (solar, nuclear, wind improvements), cheaper to store (batteries), and more efficient to deliver as power (motor designs, integrated computers). A combustion vehicle has in itself finite efficiency, and the only improvements you can make apply to EV's (cheaper power generation would lead to less costly delivery of fuels).
An interesting bit, even if power become free to generate, transporation of it costs around four cents a kWh. Since solar and wind can be localized better, especially with batteries, it doesn't matter if production were free for another source if the cost per kilowatt hour is brought down enough.