Can only concur with what Sussex says about the 1.0, 3-cylinder petrol engine - presumably the VAG one, which I've got in my Octavia.
So mine's is a petrol, and automatic, yet roughly speaking I get 20% or so better MPG than in my old VAG 1.6 diesel manual
So if I was getting 30MPG in my old manual diesel, I'd be getting 40MPG in my auto petrol, if getting 40MPG in my old manual diesel I'd be getting 50MPG in my auto petrol etc, etc.
Two caveats, maybe - if up to motorway speeds then the advantage isn't the same, and in fact roughly speaking I'd say they're broadly equal at 70mph, or whatever. (Although I'm not normally driving at speeds like that, therefore...)
The other thing is that when the aircon is on in my new petrol I'd say it has a much more noticeable impact on MPG than in my old diesel.
Power wise, my old diesel was quite small and only 105bhp, and the 1.0 petrol is actually more powerful on paper at 110bhp
Although unsurprisingly the torque is a lot lower in the petrol, although I can't recall the figures offhand.
Of course, the 1.0 petrol isn't going to win any boy racer competitions, but nothing wrong with it at all in terms of shoving the car along in day-to-day driving.
But because it's an automatic with the 7-speed DSG gearbox, and because the fuel consumption is helped by the cylinder shut-down/coasting function, I'm certainly wary about long term reliability and costs
Although from what I've been told, and again concurring with Sussex, the 1.0 VAG engine is very reliable on a long-term basis. We'll see...