Engines - IMHO - The Aygo KR-FE unit is very much an up to date 3cyl petrol, probably better for compactness and lightness than excellent torque / performance. In my Aygo the gearing is about 22 mph / 1000 (based on sat nav) in 5th, but you do feel the air resistance at anything over 60. With such a streamlined shape maybe a longer geared and higher torque setup would be better.
The equivalent Hyundai engine is the Kappa 3cyl from the i10, similar spec to the KR-FE but heavier, but does have a start stop and intelligent alternator system - reduced load on hills for example.
What is the target weight for this car ?
The FIAT Twin Air is good on paper, it has good performance for the size especially. Reports "from the field" though seem to suggest that even in it's low-boost "eco" mode it can't match the claimed economy figures and isn't as good as the 1.3 Multijet Diesel.
General: Twinair - Rubbish mpg with City Driving - The FIAT Forum
Fiat 500 Twin Air MPG - myth or con!
Twin Air MPG
General: TwinAir - real world consumption - The FIAT Forum
The 1.2tdi from the Lupo 3L / Polo Bluemotion may be a better bet if you want the best torque / economy compromise. Heavier than both and would need more space but a great engine all the same, and has the option of a 6speed.
There is the
1.2 TDI Insight as an example of course. And
Vekke is aiming for 2l/100km from his.