Is the Fiat 500 really economical?

Wig

OP needs to first work out what she is getting on fuel consumption.

Fill it right to the top (not half a tank). Ignore this fill up throw this receipt away.
Reset the milometer
Drive for 4 weeks filling up as required - keep your receipts as a record
At the end of the 4th week fill right to the top again add this receipt to all the others.

miles/gallons = fairly accurate mpg

gallons = litres/4.546