3 motoring posers for a summer day

I joke about this, of course, but I truly would like to know what I’m supposed to do when I see such signs. There must be a logical explanation, even if I can’t think of one.

For some fun this week, we present three driving questions that are bona fide head-scratchers. Kudos for knowing the right answer to any of them.

Or do the signs simply warn motorists that debris might fall from the truck, and if you value your paint job, it would be a bad idea to follow too closely?

For the correct answer I went to Sergeant Tom Fitzgerald, a veteran of the State Police commercial vehicle enforcement section. To my surprise, Fitzgerald told me that none of my guesses were correct.

“The intent of that is to not follow the dump truck into work zone setups,’’ he said. “You’re out there on a highway, and traffic is stopped. The truck all of a sudden takes a left turn to the other side of the cones. And you might be thinking, ‘Well, I guess it’s OK to follow him. He knows the way.’ Well, unfortunately his way is the paving machine.’’

So if you’re on the road, you can drive the standard distance behind a truck bearing the sign?

“Sure, if you want to,’’ Fitzgerald said. “The whole point of it is not to follow the trucks into the work zone.’’

So I asked Carroll a question that’s been burning inside me for years: Why do gas stations tack on 9/10ths of a cent to the price of a gallon of gasoline? How the heck are you supposed to pay 9/10ths of a cent for anything?

“You want to know why the 9/10ths? Everybody asks me that question,’’ Carroll said. “I honestly believe it evolved going back to when they first started using petroleum for motor fuel, when the price was so low that they priced it in mills. A mill is a tenth of a cent.’’

He said stations are not required to charge the 9/10ths. But old habits are hard to break. “I think it just evolved through the evolution of pricing and it was just something that was maintained in the industry,’’ he said. “They kept it because instead of saying $1.90, they say $1.89.9. It’s a psychological thing.’’

What if you wanted exact change back?

“I don’t know anyone who carries around mills in their till,’’ Carroll chuckled. “The computer rounds to the nearest cent.’’

As silly as all this is, Carroll said that stations are required by state regulations to post the full price of gasoline on signs, including the 9/10ths.

“We have had some problems where they’ll put a price sign up at $1.89 and they don’t put the 9/10ths up. We go in the station and we find out they’re selling it at $1.89.9. That’s a violation. We hit them with a fine and make them straighten their signs out.’’

I caught my mistake after pumping about 2.5 gallons into the tank, but I nearly panicked over the trouble I might have caused. Would my car run on diesel? Would diesel harm my engine? Did I have to somehow pump it back out?

In the end I said a prayer, topped off with gasoline, and drove away. Later, I asked John Paul, AAA of Southern New England’s “Car Doctor,’’ whether I’d done any permanent damage.

“Diesel is essentially less-refined gasoline. So it’s a lower octane, and the fuel quality isn’t as good,’’ he said. “Would you damage your engine? Probably not. In the worst case it might not run.

“Chances are it’s going to smoke a little bit and it might not run the way it did before, until that mixture is out of the tank.’’

Paul said I could have filled the tank with 93-octane gasoline to try to offset the low-octane diesel. If I had pumped a large amount of diesel into the tank, it would have been advisable to have pumped it out and to have changed my fuel filter, he said.

Diesel nozzles are normally fatter than gasoline nozzles, he added, so most of the time motorists are prevented from making such mistakes.

“The other way around, and you added gasoline to a diesel engine, that’s where you can do real damage,’’ Paul said. “Gasoline is much more explosive than diesel, and diesel engines are much higher compression, so you might do some real destructive damage to the engine.’’

Peter DeMarco can be reached at demarco@globe.com or on Facebook at “WhotaughtYOUtodrive?’’