I didn't really want to get into this thread because there are certain contrarians that will argue one thing one week, then argue the opposite the next.
This is the most ridiculous advice I have ever heard. You are so obsessed with the difference between real world mpg and claimed mpg, that you forget about the most important thing in life: You can spend your money only once!
For the price of the cheapest BMW 316d I can buy a very, very nice 500 with TA and enough petrol for driving 150k miles. So why care if the real world mpg of the 316d are close to the advertised? Unless the 316d doesn't use fuel at all, the 500 with TA is a much better investment!
Andy Numpty said 'large' TD BMW.
https://www.honestjohn.co.uk/realmpg/bmw/3-series-f30-2012/335d
Gosh, said Numpty, 74% sure is a lot better than 71%
https://www.honestjohn.co.uk/realmpg/fiat/500-2007/09-twinair-105
If you do the maths, a 500 105 will cost you 1.68l/100km more than advertised. In CO2 that's a .35 tons per 10,000km
The 335d will cost you a staggering 2.2l/100km more than advertised. In CO2 that's an even more staggering .57 tons.
What's even worse is that a small city car doing short trips is always going to struggle 'real world' vs a big diesel that's much more likely to be doing long journeys. Like for like that delta will only get worse.
Numpties don't realise that MPG is a measure of diminishing returns, can't understand basic math, nor do they ever acknowledge that diesels put out much more CO2 than petrols - they just like to argue.
Certain Numpties seem to think 70mpg official, 50mpg real world, OMG that's 20mpg! A V8 Range Rover uses 20mpg! A Fiat 500 uses a whole Range Rover's worth of fuel more than it should! In fact Numpties, it's costing the owner the same as a 20mpg car getting 18mpg. 'Only 2mpg off, that's pretty good' mused Numpty.
The number of faults listed on Honest John for the 3 Series is pretty staggering for a 'premium' car costing what it does.
The really funny thing is that of course, diesels only match their ratings when driven extremely gently. That EE video also describes every turbo diesel in existence, while petrols may add fuel under boost (say going from 14.7 to 16:1), it's nowhere near the level of diesel (going from around 100:1 to 25:1). My diesel gets it's best economy in gridlock, pity it drops like a rock if keeping up with regular traffic.
Overall, downsizing works 100%. UFI was such a 'disaster' I went on to two even more drastically 'downsized' engines:
Fiat 500 TA= 875cc/ton (incidentally, ever wonder why the odd capacity? - it's actually the thermodynamically 'ideal' cylinder size). I get 50-70MPG. Inspired by this discussion I drove it like I stole it for 50km in the hills, couldn't get it below 50mpg. You'd have to be truly reckless or live in a place with no speed limits to do worse. Best I've seen is 70MPG (suburban)- tank averages are in the 60 range. If anything the HJ real world figures sing the praises of the TA105- same economy as a 1.2 with 30% more power for free - I only wish the 105 had been sold here.
Renault 1.6 petrol= 0.666cc/ton. Worst peak hour city commute was under 30mpg - seriously impressive for 2.4 tons and a .45Cd. Lifetime average was 35mpg, 99% city/cbd use. Much better than anything that heavy should do - certainly better than the 2.3 petrol (24mpg!) and 2.4 diesel that I drove prior.
Renault 1.6 diesel= 760cc/ ton, best ever cbd commute of 60mpg. Not bad for 2.1 tons! Tank averages 38-42mpg. Way better than the 2.5 and 2.4 (28mpg) diesels before it.
I've been on drives with enough 'enthusiast' Numpties to know not to care what MPG they get. They nail the throttle in a straight line and slow to walking speed at the slightest hint of a corner. Non-enthusiast Numpties can't anticipate three feet ahead of their bumpers, leave their cars idling for hours on end (literally saw a guy idling for 2 hours in a car park the other day) and never so much as check tyre pressures. Why should I care what the Numpties get?
If I'm researching a car I'll look at the top 1 or 2 cars I find on fuel reporting sites - that's what I can expect to get.