40MPG ??

First off, there's no reason why older designs can't return decent consumption. Real world fuel consumption for many classes of vehicle hasn't changed much in the last 30 years, under some circumstances and like for like. You'll probably find that a late 1980s 2.0 Sierra will return broadly similar consumption to a late 1990s Mondeo 2.0 to a current 2.0 Mondeo. Sure, there have been drivetrain improvements, aerodynamic refinements, taller gearing, better oils - but there are also much higher weights, more economy-robbing emissions control gubbins and wider wheels and tyres.

I'd expect a current 2.0 Mondeo to be capable of lower consumption than a

2.0 Sierra under optimum conditions, like, a flat motorway route at 55 mph. That's why I slapped in "real world consumption."

Second point is that the larger and heavier the car, the greater the difference between driving it harder and driving it gentler is. That's because of weight and gearing. It takes a lot more power to accelerate something that weighs 1,500 kg to something that weighs 1,000 kg. Drive something big, heavy and powerful* but be gentle and it'll reward you more than something lighter, smaller and less powerful*.

Pottering around in the upper gears between 50 to 70 tends to be economical unless you start labouring up gradients and suchlike. Adopting the truckers' lope in the inside lane could potentially save a stack of fuel, too.

*Here I'm comparing a 1.8 Sierra with a same-age 1.1 Fiesta, not, the Veyron with a Smart.

Very much depends on the circumstances and driving, though. 24 to 27 in the city may not be comparable to your driving at that time.

My knackered ancient A40 used to do 40mpg in the early 70's, my runabout metro 1.1 now does about the same, but is a bit faster, I wish I still had the A40 !! Great little car.