The hypothesis: that the fuel economy of one of those new small petrol turbos is as comparable to that of an equivalent diesel in reality as it is on paper.
The experiment: first take two versions of the same car and compare them. I don’t have two versions of the same car but my father has had. He replaced his diesel Focus (1.5l, 99 bhp) with a petrol one (1.0 turbo triple, 125 bhp) in 2018. I never drove the diesel but due to the Current Unpleasantness I’ve been driving the petrol one for the past few weeks.
This is what it’s averaged in my hands and on short journeys:
7.7 l/100 km is equivalent to 37.7 mpg (Imp) or 30.5 mpg (US).
As I recall the diesel did just under 6 l/100 km which equates to 47.1 mpg (Imp) or 39.2 mpg (US)
I find the hypothesis unproven.
Yes, it was only 14 deg today. Yes, it’s supposed to be summer.