Yes, actually, the 3800 series in GM "full size" cars was very economical for the size of the car. If you are looking at used cars, perhaps a 2005 Chevrolet Impala (or similar.......) it would get you 33MPG on the highway (really)....and low 20's in town. Their replacements (3.5 or 3.9L V6's in today's Impalas) actually are less fuel efficient. I know, I had both....the 3.8L was around a hundred yrs. it seems, but they got it, with OD to produce great mileage. GM's direct injection 3.6L, V6 platforms are nice, but if mileage is imperative, then see their 4 cyl. choices below.
Not exactly sure how you blend the occasional "engine transport" with a "car". If you handle the engine w/a trailer or truck, then there are a wide variety of cars that will get you good MPH. The Jetta TDI is a good suggestion for mileage, not sure I want VW reliability....not so good, statistically.
Ford Focus, while a little mundane, fits well and is economical. Fusion Hybrid?? Very good (& nice) car w/great mileage. Or perhaps a Focus w/4cyl. IC engine? It's the new Motor Trend Car of the Year.
GM products with Ecotec 4-cyl.'s get 30+MPG highway (Malibu, new Equinox SUV, etc.). Malibu is a reasonably sized car.
Of course, there are always the super small vehicles, the Honda Fit, Toyota Matrix, etc.. Not sure how comfortable they are for lots of driving w/two people & what ever you carry.
One other car that is quite good and perhaps better last year than this 2009, is the Mazda3. Comes in several forms......big inside for a little car, lots of good reviews about how well it handles. Comes with two engine choices.....but, mileage apparently is better in other cars in the segment. For small, that might be my go to car, if new or slightly used is an option. You can buy an automatic or manual, depending.
Just a few suggestions..........................
See ya.
I would recommend a GM with the 3800 motor. If you get a Lesaber or Bonneville they have enough interior space and can haul a fair amount in the trunk.