Here's a review on whatcar.co.uk of actual users of the VW Polo 1.2 TDI 75 (80 ?) hp 3 cylinder diesel.
Note, these are all imperial mpg figures.
"This is a car built for a purpose - fuel efficiency. This is what it does well. Commuting I am getting 70mpg, however on a longer run, I achieved 79MPG on a 550 mile drive through scotland, the motorway portion of this I calculated as being 89MPG. This wasn't going too fast and constant speed on cruise control.
The downside of this is that the car can be quite slow, especially when accelerating uphill. This is a small price to pay. It can be nippy if you go into higher revs, but this wouldn't be fuel efficient."
This is for the VW Golf 1.6 105 hp diesel.
"Seen high 60's on the motorway on c/c, but lows of 25 in town. Better than the 130hp Laguna, but not what was promised."
Now I'm always suspicious or reader's mpg calculations, and the "I get xx on the Motorway" what matters is your average per tank not what you get on a particular trip. So I calculate at the same pump. I fill to the click and round off and calculate, I don't do a brim to brim as that's a pain in the arse and we're looking for average not exact.
Now I couldn't drive a slow car such as a 3 cylinder diesel, never. I'd rather have an 8 year old car that's faster and convert to lpg, people rarely calculate depreciation as a substantial cost of car ownership which in many cased adds up to a whole lot more than fuel costs.