I'm not really sure you can rationalise this. I take a bike on a trip because I want to go by bike, not because it is cheaper or more convenient or any other reason that would stand up on a spreadsheet. There are also times when I don't want to or can't go by bike so I drive or fly or take a train. Just regarding a bike as a cheap version of a car and only justifiable in economic terms is the first step to giving up riding - in my opinion.
There's no doubt that small diesel hatchbacks are phenomenally good on fuel these days and probably better than any bike over about 200cc (slogging singles excepted) but unless money is really tight there has to be more too it than that. Otherwise nobody would put up with rain, wind, cold, stupid service intervals, minute service life, verging on criminal dealers, legal obstacles, insurance costs, theft, vulnerability, public perception and all the other downsides of life on a motorcycle. And that's not even mentioning fuel. I'm sure one of the big manufacturers could produce a 100+mpg diesel bike without too much effort but who's going to buy it (in sufficient numbers to make it viable anyway)? Not many of the people buying 70mpg diesel hatchbacks.
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