Do's and Don'ts
Was going to post identical details to Greengopher but I don't need to now.
Telstar/626 are very heavy on fuel. If you want a car just for fuel economy you really have to find out reasonably accurate consumption figures.
Most carbed fours that would be in the ballpark, price wise, at best will use only 15% less fuel if you have a V6 or 25% if you have a V8. Hardly worth it given the reliability of the unstressed Commo motor.
The exceptions would be the Barina/Swift or Charade but the fours only.
In these cars the 4cyl has almost identical economy to the 3cyls and in the Charade the 4cyl is just shy of double the power - 66kw to 37kw.
The Charade gets along very well 0 - 100 in 9.5seconds, excellent handling and close to 50mpg (I used to own one).
The only other cars that have lower than average consumption in their class is the old 1.5 or 1.6 Astra/Pulsar. Up to 45mpg, very cheap but also very flimsy. Do not get an Alfa as $1500 would be your monthly bill just keep it going and to stop it falling apart.
The best idea would be gas. I run gas on my VN and for $25.00 I get 440 - 480km. With petrol at $1.30 it is the monetary equivalent of getting 70 - 75mpg (=26km/l or 3.85L/100km if same distance travelled on $25.00 petrol).
No four will give anywhere near that. Then again, you could buy a four on gas.