54 is only viable with carbon fibre or other lightweight manufacturing that is not currently viable cost wise.
To get a 50% increase you have to decrease weight, its the only viable way.
Leftists can't say it but weight is not what concerns them, it's size envy. They want your car 50% visibly smaller than theirs. Increasing MPG is just the socially acceptable cover story, and why a cost/benefit analysis isn't a factor.