Son and wife both have Jaguar XE cars, his however is 4 wheel drive, but same route and same speed both cars get the same MPG in spite of his having an extra 30 HP. What however makes a huge difference is tyre pressures, and tyres.
It is like when we see a fluorescent lamp we all know with a HF ballast the tube gives out more light, lasts longer, and uses less power, but it always shows the tube wattage with a wire wound ballast even when now discontinued.
Seem to remember her car has a selection of driving modes, sports, normal, eco, ice etc. This clearly alters MPG, and even if not an automatic how fast one accelerates and brakes is going to affect how much fuel it uses.
I would agree the manufacturers figures some times seem out, we had a Toyota Yaris and a Vauxhall Agila at the same time, wife had gone to USA to study, and I was working on building of T5, so I alternated between the two cars, about same size, about same power but Vauxhall was manual and Toyota automatic, both showed same MPG in the paperwork, the Vauxhall was always just under 50 MPH and the Toyota just over around 54 to 56, the Toyota was all round the better car, but also cost more to buy.
What seems daft is we get better MPG from the Jag to either of those cars, mainly due to being diesel, but also having 8 speed gear box, and to be frank if I had an 8 speed manual gearbox I don't think I could select the best gear for economy anywhere near as well as the car does. Also the towing weight for manual 1500 and auto 1800. Once we go electric no option but automatic, but the old 3 speed auto box is very different to the 8 speed used today.
Yes I have driven wagons with 24 speeds, and automatic wagons with 16 speeds, (CAT 769) but they accelerate a lot slower, I can't see how I could use an 8 speed better than car does, OK car has paddles so can take control, but why would one want to?
But at end of day unless automatic and it states which mode is used, MPG means nothing, all down to driver.