DEAR CAR TALK: I don't understand why so many people buy diesel-powered cars and trucks. They are loud and emit horrible pollution compared with gas-powered vehicles.
What are their perceived advantages? I can't believe they outweigh the negatives. Can you please weigh in? And why are all the semis on the road diesel rather than gas?
— Hayley
DEAR HAYLEY: Believe it or not, diesels have evolved quite a bit over the past couple of decades. The people who used to buy diesel cars were mostly end-of-the-world preparers or inveterate cheapskates.
The doomsday crowd concluded that in the event of a nuclear attack, with its electromagnetic pulse, their diesel Rabbits would still be able to run without needing an ignition system, and they'd be able to go to the 7-Eleven and buy iodine pills and lord it over their neighbors.
Cheapskates liked diesels because, despite their noise, vibration, smell, inability to start in cold weather and black exhaust, they got higher mileage than gasoline cars.
Trucks have traditionally used diesel fuel for those reasons, and others. Because diesel engines operate at high compression, they have a lot of low-end torque, which helps a 30-ton tractor trailer pull away from a traffic light.
Plus, before diesel fuel was required to be low in sulfur to reduce pollution (it used to be just liquid dinosaur bones), the fuel used to be pretty cheap. So you got better mileage and paid less for it. That's no longer the case, but it was.
So why do people still buy diesels? Well, not a lot of people do. But the truth is they've improved a lot in just about every way.
Newer diesel-powered cars are reasonably quiet — at least inside the cabin. Vibration has been controlled to where it's barely noticeable. Cold weather starting, which used to be a huge problem, has been largely addressed through the use of plug-in block heaters. And even the pollution has been brought largely under control.
We know from the VW scandal that we can't trust the manufacturers to report accurately on diesel pollution, so we spoke to Gary Bishop, a researcher at Denver University. He's part of team that created a gadget that uses a spectroscope to measure pollution as vehicles drive by.
Gary set up their device on highway off ramps, and, as a car drives by, their device captures its license plate (so they can find out the make, model and engine), and the amount and types of pollution coming out its tailpipe. So this is as real-world as pollution data gets.
What they found is that newer, well-maintained diesel engines come pretty close to gasoline engines now in terms of pollution.
Diesels used to spew particulates (basically soot) into the air. But since particulate filters were made mandatory, newer diesels produce very little particulate matter. Diesels used to produce tons of nitrous oxides. But newer diesel cars come with refillable urea tanks that the emissions control system uses to scrub NOx emissions. That's apparently working, too — as long as the owner actually refills the urea reservoir. And with newer, low-sulfur diesel fuel, sulfur pollutants are pretty well taken care of, too.
It has taken decades for diesels to clean up their act. But the newer ones seem well on their way to getting there.
Of course, it may be too late to save the diesel engine in America. Diesels have a bad rap here, and, for a long time, they earned it.
But here's an even bigger, long-term threat to diesel engines: You know what else is much more economical to refuel than a gasoline engine, produces far less pollution, makes no noise or vibration, and has a ton of low-end torque? Electric motors.
Battery technology isn't there yet, but at some point, that's probably going to be the obvious solution for cars and trucks.
DEAR CAR TALK: I have a turbo six-cylinder car that calls for 93 octane gasoline. If I were to hock my stepson and kick my dog until she ran away, I could finally afford to fill the gas tank with high octane fuel.
If, on the other hand, I could safely use regular gasoline instead without causing harm to my engine or turbo, I would be able to bring back my dog with the savings.
Can I do that without negative consequences?
— Sy
DEAR SY: It depends.
This is one of those things people often overlook when buying a new car. You drive it, you see how it rides, how it accelerates, how it looks, how the seats conform to each butt cheek. But most people never open the fuel door or check the owner's manual to see what kind of fuel it takes.
If you buy a car that requires 93 octane gasoline, you're going to spend 25-40 cents more per gallon at every fill-up for the life of your car.
One of reasons some engines can produce a lot of power for their size is that they compress the fuel air mixture at much higher pressure. They're called high-compression engines.
But in order to prevent pinging (or preignition) in high compression engines, they need a fuel with a higher ignition point. That's what higher octane fuel has.
So why do I say the answer to your question "depends," Sy? Because it depends on whether your owner's manual says "Requires 93 octane" or "93 octane recommended."
If your manufacturer recommends 93 octane, that means the car is designed to perform optimally on 93 octane fuel. It may have an iota more power and possibly a hair better gas mileage when you fill it with 93. But it also means the car has a knock sensor that will adjust the spark timing when you use lower octane fuel to prevent the engine from pinging, and keep the engine from getting damaged due to overheating.
On the other hand, cars that "require" high octane fuel also have knock sensors, but for whatever reason, the manufacturer makes no guaranty that you won't damage your engine if you cheap out and run it on regular fuel.
So, if it says "requires 93 octane," then you're out of luck, Sy. As is your dog, who was probably looking forward to shedding all over the backseat of that BMW.
Next time you go car shopping, put that on your list of things to check. And then you'll know in advance whether you have to choose between the Five Series and Fido.
Ray Magliozzi dispenses advice about cars in Car Talk every Saturday. Email him by visiting: