Re: Surprisingly thirsty?
> You are comparing apples with pears.
Agreed, though it is possible to convert between the two. Diesel has about 12% more energy than the same volume of petrol, and correspondingly releases about 12% more CO2. (See, for example https://en.wikipedia.org/wiki/Diesel_fuel#Fuel_value_and_price)
For a diesel car to have the same CO2 emissions as a petrol car that does 52mpg, it would need to do 52 * 1.12 = 58mpg.
> The sweet spot for fuel consumption, if you're fuel-agnostic, is the Focus size.
It depends on the type of driving. There are three main things that cause you to use fuel. Changing momentum, overcoming air resistance, and overcoming friction with the road.
For city driving, changing momentum is the dominant cause of fuel usage, since you spend a lot of time accelerating and decelerating, and you are going slowly enough for air resistance to not matter. The way to reduce this fuel usage is to reduce the weight of the car, hence city cars are small and lightweight. Since air resistance is neglible at low speeds, they don't need to be as aerodynamic.
For motorway driving, air resistance is dominant, since the force is proportional to the speed of the car cubed* but you don't change speed much. Here, reducing the car's weight doesn't have much of an effect, but making the car more aerodynamic does. This is why bigger cars can achieve high fuel economy for motorway driving.
Friction of tyres on the road is usually smaller than the other two causes of fuel usage, but chunky off road tyres are less efficient than slicker road tyres (unless you actually are off road, in which cases having the wheels spin road without you moving anywhere is infinitely inefficient).
*ignoring wind for simplicity