We have had our Smart since July 2008. Currently have 26,000+ miles on it.
Our best mpg is 52. Our worst about 25. We quickly learned.
1) Name brand gas only. Nothing from Sam's
2) Run in automatic on the transmission.
3) Fill the tank when it gets down to half.
4) Only run Premium gas.
5) Best mpg comes staying between 60 - 75 miles per hour.
We have gone as fast as 95, run it down to so close to empty that we put in 8.9 gallons and tried driving in manual.
Don't know how others are doing but that is what we do.
Combined city - highway is usually 38 to 42 depending on the combo. The A/C doesn't seem to matter much to the mpg.
We live in the Houston, Tx. area
Love our "Baby"