This was possible not just because diesel engines burned less fuel (about 20 percent) to move a vehicle a given distance but also because diesel fuel cost less than gasoline.

Of course, that was once-upon-a-time.

Diesel now costs more than gas. It also costs about twice as much as it cost only a year ago, when the price averaged about $3.50 per gallon. It was $4.10  as recently as late August. It currently costs about $6.50 – when this article was written on Sept. 26. By the time it is published, the cost may be $7 or even more, depending on political events. The cost of that is unaffordable – for most people.

A few weeks ago, I reviewed the '27 Ford F250 pickup, which was optioned with the turbo-diesel V8. This truck comes with a 34 gallon tank. At current, late September prices, it would cost about $220 to fill this truck up. If you had to fill up once a week – as most people generally do – it would cost you nearly $900 each month, which is a sum equivalent to the monthly payment you'd be making on the F250 itself.

There is also the cost of diesel exhaust fluid (DEF) which adds to the cost of owning any diesel-powered vehicle that requires it – which is all of the currently available ones. It is not – by itself – a prohibitive cost. But it is another cost added to the cost of owning (and driving) a diesel-powered vehicle. There is also another cost. One that is hidden but nonetheless one everyone who buys a diesel-powered vehicle pays. It is the cost of the DEF system – which includes the DEF tank, the DEF quality sensors, the DEF pump and lines, the DEF injector and the diesel catalytic converter(s) as well as the cost of maintaining/repairing these parts.

There are other parts, too.

Diesel engines used to be less costly to operate because they had fewer parts than gas-burning engines and so there were fewer things that needed to be fixed. For example, diesel engines – when they cost less to operate (and even to buy) – had a relatively simple mechanical injection system that required no electronics (unlike the electronic fuel injection used in gas-engined cars since the 1980s). I own a diesel-powered tractor that is nearly 50 years old and its mechanical injection system still works; in the 20-plus years I have owned it, the only maintenance I have had to do is change the fuel filter and (once) purge air bubbles from the lines. It does not need maintenance, basically.

The mechanically injected diesels once used in road-going vehicles are (were) similarly low maintenance. They have no spark plugs or wires; no distributors or coils, either. Some of the older ones don't even have turbos. They ran kind of like a perpetual motion machine – via heat and compression – once you got them running. Which you could do by roll-starting them if they have a manual transmission. You don't even need a battery – or an alternator, which they have but which is there only to keep the starter battery charged up. Once one of these old-school diesel is running, you don't need the alternator (except to run the lights and other electrically power accessories).