My dad had an old diesel AX as a replacement car back in 1990, and it was so economical he'd forget to fill up.
BTW, have you seen this?:
Peugeot 205 5th gear swap (8.2% improvement at constant 50 mph) - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
It's about a 205 but the owner talks about his own Citroen AX.