I've had two 451 smarts, first a 2008, now a 2012. I record fuel mileage on a computer spreadsheet, noting the gallons from the pump and distance from the trip odometer. Cumulative fuel mileage for the first one was 33.6mpg, and for the current one has been 34.96mpg, each over a span of ~12k miles. Both of them have achieved 42.5 a couple of times on the highway. But most of my driving is in town with 20 and 25mph speed limits, only rarely seeing 5th gear, lots of stop signs. The car is really great for local errands but clearly this is not ideal for fuel mileage!
That first 42.5 was encouraging as it was only the second day of ownership. I had bought this new car via eBay and picked it up in LA. Driving home I measured the fuel consumption from Redding CA to Yakima WA by way of Klamath Falls and Bend OR, involving a lot of steep climbs and higher altitudes. But moderate speeds, 55 limit much of the way.
My insurance agent had one for a while and commuted 6 miles to work in town on rural roads with a 50mph limit. He says he drove like a maniac and got 50mpg all the time! I suspect the clue to his success was the steady moderate speeds.