As has been pointed out before, modern cars after the VW mpg scandal are tweaked to make mpg the be all and end all, witness the hesitancy of VWs at junctions (engine starved of fuel for acceleration to improve mpg) so how bad would MPG be for most cars if they weren’t tweaked. Two factors come into play too, cars get bigger and bigger, engines get smaller and smaller. Yes at peak condition in ideal circumstances they might be great, but how often do those conditions arise? If not then you have the classic big car small engine problem. I like diesels, always had though I have had great big 3 lite v6s. I like the economy and grunt of a diesel that without tweaking and tuning can get stellar mileage and ultra reliability. Oh for the days of the non turbo diesel. Slow but reliable.