Maybe someone can clear this up for me. Given identical levels of maintenance and identical (very gentle) driving technique, which Polo will burn the least fuel over an identical urban/motorway cycle?

1/ an MPI 1.3

2/ a GT

3/ a tuned GT

What I'm trying to establish is if the increased performance available to 2/ and 3/ isn't used, are they more or less economical than 1/?

I don't think there's much difference between one and two, having owned both as daily drivers for at least a year each. The non-gt is a bit better on motorway though because of the longer ratio gears (GTs rev a bit high on the motorway), should still get 40mpg either way.

3 isn't going to be as economical, the whole idea of tuning is to make the engine able to burn more fuel! But really, it depends on what you mean by "tuned", up to 100bhp you probably aren't going to see a huge difference in mpg with normal driving.