38 for the majority of vehicles is the upper end... you don't see a whole lot other than hybrids or really crappy little ratboxes going past that. Many so called 'economy' vehicles like the GM ecotec engine ones actually get worse mileage than their larger cousins because the engine is too small and underpowered. (common myth, less power == better mileage, enjoy burning twice the gas getting up to speed)
Back in the 80's we were pushing 35 city and 50 highway before all the ecof... f.... folks (Trying not to piss off Wanda Sykes) got the BS of emissions control in place. Nothing like crippling the MPG to save the environment "Oh, emissions are down 30%" -- really is that why I have to burn twice the gas to get anywhere? Makers right now are struggling to keep up above 30 city when that's less than we had 30 years ago... and that's before the real lie of MPG estimates.
They keep changing how it's measured so that the numbers climb up... which is why if you measured most cars by the 1980 standards they'd have less MPG than a pre-emissions controlled pre-nutjob scale safety standards Plymouth Cricket. (that's Hillman Avenger for our friends across the pond)
Take a physics course. That would require a level of efficiency even a turbine can't achieve, and a power to weight ratio that would mean the car would have to come in at under 150 pounds with the horsepower of todays 2500 pound car.
As the hybrids have shown... Or just motorcycles, which have seen a decrease in mileage due to emissions but can still push past 50mpg due to their power to weight ratio and total lack of safety equipment. 250CC or smaller can push past 70, but that's rare at best... while a 50CC can pass 100 -- but the ones that do aren't street legal (since you can skip a whole slew of heavy gear when building a dirt bike). The majority of motorcycles produced today not getting as good a mileage as a '39 Nimbus or a pre 70's oil crisis Honda. Many of your 500's and larger don't even compare well to automobiles!
There is only so much energy in a gallon of gas; unless there's some miraculous super-material breakthrough it's physically impossible for any car over 1000 pounds to break 60mpg and still pass safety standards... or have the struts punch through the body when you hit a speed bump like a 1960 lotus elite. Power to weight ratio -- it's not just about speed. See why a Chevy Malibu gets as good if not better mileage than a same year Cobolt or Aveo... or whatever they're calling their latest economy ratbox.