Reality is that your MPG numbers are going to depend more on
how you drive,
where you drive, and
how long you drive than anything else. Short trips, quick starts and stops, and overly hilly environments will sap the MPG in
any car. The EPA rating gives 2 numbers as markers just for this reason, one for short-trip intervals (city) and one for long-trip intervals (highway). Plus, for new cars you have the break-in period where you'll get under "normal" performance anyway.
The smart is the most efficient gas non-hybrid car on the market. If you had any other gas non-hybrid car in the same conditions
your MPG would be lower. If you start throwing in EVs, hybrids, CDI, CNG, and the likes, then all bets are off. But for a gas car you'd be doing worse mileage in anything else. Just something to keep in mind.
For example, I live in a mildly hilly area of upstate NY, drive about 70% city, and drive on snow 5+ months of the year. My old 2001 Saturn got about 26MPG with the same driving patterns. My smart is getting:
Enough said.