We all have opinions, finding the facts is the hard part
Regardless of manufacturers' hype, we all know that we never get from our cars the mileage they are claimed to achieve.
The thing is that up until perhaps the mid 2000s, I did get the mileage claimed. I was able to achieve figures perhaps 10% better than those claimed with just a little effort. A side effect was (is) reduced tyre and brake wear - for some years in the early 2000s my dad and we had the same model car at the same age, though his was petrol and ours Diesel. We changed our front tyres literally half as often as he did, and the brake components likewise. We had been expecting the Diesel car to be harder.
We still have a 2006 Diesel Kangoo. It now has 128,000 miles on the clock and achieves between 55 and 60mpg regularly with a journey mix that is probably 80% "short" (under 5 miles) journeys. I used to commute in this vehicle, 45 miles each way, mostly motorway and in the last summer before I "retired" it to the school run, it did three tanks in a row, each one well over 70mpg. Its official figures were low to mid 60mpg from memory.
The car I use now (a 2011 model) has official motorway figures of (IIRC) 72mpg yet I struggle to get it much over 65mpg.
diesel fuel should cost 11 or 12 per cent more than gasoline, because it contains that much more energy and that much more carbon.
But Diesel has (since the 1990s anyway) always been more expensive than petrol. Perhaps not 12% but often 5 to 8%. The current situation where they are about the same price (UK) is unusual and I don't expect it to last. We have bought Diesel cars for the last 10+ years because even with the price difference and the higher up front cost, the economics made sense.
We must not forget CO2 emissions. NOx and CO2 are two completely different things and have utterly different reasons for wanting to reduce them. We must not concentrate on one and ignore the other.
It is only recently that hyper efficient petrol engines have started showing. At the moment they seem to be confined to very small capacity, high revving, highly turbocharged units that must be utterly tiresome on the motorway.
I would love to buy an electric car for commuting and power it with "green" electricity, but apart from the Tesla cars - which are way, way out of my reach - not one of them could guarantee to do my 90-mile commute. Yes they all claim 100+ mile range (I think the Nissan Leaf claims 120 miles at the moment) but when you start asking about the range at motorway speeds, in the winter, with the wipers, heaters and lights on then salespeople get very evasive and manufacturer websites are completely silent. I'm pretty certain one of them could get me to work but I'm just as certain it couldn't get me home again without recharging, and although we do have a charging point at work, I could hardly hog it, and what if someone else needs it?
On top of that we have relatives that live 150-ish miles away. We visit them several times a year, but turning that 3-hour journey into a 2-day journey, plus hotel costs, just doesn't work.
The alternative is to buy an electric car for the school run, but that would mean I'd have be commuting in the large family car we'd need to buy in order to do the holiday trips, which wouldn't be at all efficient.
I'm sticking with Diesel for now.