Get this, my BIL who is in the used car business - specializing in German cars just recently sold a used, I think 2000 or 2001 Golf TDI with 444,000 kilometres on it for $7,500.00 CDN. Unbelievable. Apart from looking for a used VW TDI which are rare as hen's teeth now, I have to agree with Bird's recommendation on a used Escort or Protege clone. Generally reliable and benefits from lower residual values compared to Honda, Toyota which in my opinion are overrated.
Around here, the hot commodity right now are used Audi A4s with 1.8T. Premium performance oriented sedan with amazing fuel economy. Or how about a full size all-aluminum Audi A8 with 345 HP V8. Gets well over 25 mpg US on the highway and sounds awesome. These underrated cars that listed for roughly $100,000 new, go for around 20,000 at auction for a 2004. When you are shopping used, massive depreciation is your friend. Let someone else take the hit for the first 3 or 4 years
Of course it is the overall cost of ownership that one should consider, not strictly fuel economy, in which case it would be hard to beat a Cavalier or 3800 engined Impala, le Sabre, or a Suzuki Aerio, Nissan Sentra or a 2003 and newer Ford Focus.
BTW, my BIL had a used SMART. A nightmare of a car. was prone to dying in the middle of the road, or the tranny would just decide to shift into neutral while driving. Was rife with electrical bugaboos. Apparently the battery box and other electronics modules are located behind the right-hand side inner fender, right where ice, snow, road crap will accumulate. Ran it back through the auction and lost money on it. Win some, lose some.