Are you kidding? So many things, so little time. It's not just the engine (though it counts, too), it's also the car it's in. Some thoughts -- not claiming to be an exhaustive or complete list:

Engine: designed to be low output (what used to be called detuned), to be understressed and trade power-output for longevity and reliability (barely more than 100 hp out of slightly better than two liters is an embarrassment compared to the cars nowadays)... its big bearings are an example of this, as are the ports (not intended for high pass-through of volumes of air and exhaust) ... i.e., not good for fuel efficiency, but that wasn't the biggest concern when the red-blocks were developed. The pushrod red-blocks, B16, 18, 20 and 30, were worse -- I had two B30s (164s), and they drank fuel like it was only water (15 mpg was the norm) -- but gas was under a dollar, so who cared?! The newer OHC red blocks are better, though not in the league with ricers.

Then take those low-output engines and put them in a brick (literally, and the pun is intended). No streamlining to speak of. That's a lot of air to push aside -- refer to the "Bosch Automotive Handbook" (any edition) to see some graphs showing the effects of streamlining (or lack of same) on the energy required/consumed to maintain highway speeds.
And consider the weight of the car -- from all that aforementioned metal -- which, by the way, is there to (1) protect you [a selling point] and (2) to hold up to years of abuse [another selling point]. The 240 is quite heavy (~3,200 lbs) compared to ricer cars. That weight comes from being over-built, with heavy gauge sheet metal, burly boxed sections that resist deformation in the passenger area and only allow deliberate collapse in crush zones -- these take weight, especially in the years prior to modern high strength alloys.
And if you want an underpowered car to propel such a heavy, unstreamlined vehicle, it helps if you've got torque to accelerate it, so you put in a low axle ratio to boost the torque to the wheels. Only that, too, is not good for gas mileage.

But just be thankful you've got a top overdrive gear -- I owned two Mercedes sedans of roughly the same age as my current 240s: a 190E2.3 (M102 motor) and a 300E (M103) -- the 190's top gear ran the car over 500 rpm faster than my 240s at the same highway speeds (and had about 25 more hp from the same size engine, though I can't remember the torque); the 300's top was just a few hundred rpm faster, but it had a 3 liter engine and about 70 more hp (again, I can't remember the torque). In general, MB's trick for getting "wow" acceleration was using a really low axle ratio -- to avoid wheelspin, however, the transmissions defaulted to 2nd gear starts unless the driver resorted to certain steps (to get the "wow" effect).

But you know what? The 190 weighed about 500 lbs less than my 240s; and the 300E, a much larger dimensioned car, weighed about the same as my 240s. And as a result, both cars were absolutely horrible in the snow, and even in rain (even though they had great tires, Pirelli P6000 in summer and studded Nokian Hakka 2s in the winter); all the weight in both cars was heavily biased in front, with practically no weight in the rear!

Final thought: you didn't get a 240 because you thought it would be a fuel-efficient car, did you?

Yet it IS an economy car, of sorts. Since when has gas become the only car expense? What about loan payments and (in some states, car insurance). Owning used 240s have meant that I haven't made car payments in over 20 years (after I stopped buying only new cars back in the 1970s)! Used 240s have been fine with me -- and no payments on car loans have meant that I not only have more money to spend on gas (accepting the lack of high fuel economy) as well as on boat expenses, private school and college tuitions, and in general, having more fun. So isn't that an "economy" of sorts? Ha, ha.