If you’ve seen this week’s magazine, you’ll have seen, among the plugs for next week’s issue, a trailer for a comparison test between the new Porsche Panamera Diesel and the original, genre-defining diesel sports executive saloon in its latest guise – BMW’s 535d M Sport.

I’m not about to reveal the verdict. What I would like to share with the class, however, is something I learned on the drive out to Germany for the test. Something that I think is genuinely staggering.

It turns out that you can’t set the cruise control on a BMW for any faster than 110mph. UK drivers wouldn’t know that, but you might if you spent a lot of time on derestricted autobahns in Germany. At any rate, trust me: I tried setting the thing for 154mph, and it just wouldn’t let me.

Why? Because I was intrigued to find out how much capacity a 295bhp 535d would have in reserve at the big cruising speeds you can only maintain in the country where it’s made. And there’s an easy way to do that: find a stretch of derestricted road that’s empty and straight enough, get the car up to speed, set the cruise control, reset the fuel economy reader on the trip computer, and watch what happens next. It won’t give you a scientifically accurate figure, sure - but an indication’s interesting enough.

So, 154mph being out of the question, I settled for 110mph – a speed that’s almost boringly easy to hit, and to keep, in a car as quick as a 535d. I set the cruise control, reset the economy reader, and then managed to cruise for seven miles, climbing and falling, around bends and past trucks, wondering all the while when I’d have to brake.And when I did slow, the readout on the trip computer had settled at a frankly ridiculous 26.9mpg. 26.9mpg? That’s what you get out of a M3 if you tickle it along at 65!

Which is incredible, isn’t it? In theory, this car could travel 414 miles, in under four hours, on £98-worth of diesel. On the empty, derestricted motorways that only exist in the UK in my imagination, that’s London to Edinburgh, for a tenner less than a weekday train ticket, an hour quicker, without stopping at all, and driving your own 300-horsepower BMW saloon. Eat your heart out Branson.

As you can probably tell, I like this car. To me, it represents everything good about BMW at the moment: it’s got the most convincing blend of performance and efficiency of just about anything I can think of.

An XF might be a better all-rounder, but I bet it can’t pass the Virgin Trains East Coast Mainline test.