The question of whether or not you can damage a car that asks for premium gas by using regular gas is one that never seems to die. I’ve seen this question tear families apart, with sobbing and recriminations—it’s not pretty. That’s why I reached out to an actual fuel systems engineer to get to the bottom of this eternal question, once and for all.

The question is likely to come up even more frequently now that the price gap between regular and premium gas seems to be widening. The gap is about 50¢ at the moment, which averages out to between $6 to $8 per fill-up, more, of course, if you have a bigger gas tank. That’s a good bit of money every time you put gas in the car, so it’s easy to see the temptation of the lower-octane gas.

Advertisement

First, we should explain what the difference is between regular and premium gasoline. It all comes down to octane. Without getting into all of the complicated chemistry, you can think of the octane rating of a fuel as how much that fuel can withstand a vigorous compressing before it ignites, like diesel fuel. When fuel ignites in a gas motor not from the spark plug ignition, but from compression, that’s called knock, and it can cause severe engine damage.

The higher compression engines tend to make more power and, because those pistons are squeezing that air/fuel mixture so fiercely, they are more susceptible to knock, which is why those engines prefer premium fuel.

Advertisement

Putting high-octane fuel in a lower-compression engine will have no effect other than separating you from your money. It won’t make your car run any better or worse, so if you give your non-premium requiring car premium fuel as a sort of treat, you’re just giving it a big fat placebo.

Advertisement

Modern engine computers and management systems can compensate for a lot of things, including not getting the proper octane requirement in fuel. Does that mean modern engines can’t be hurt by using a lower-octane gas than required?

To get to the bottom of that, I reached out to GM engineer who works with fuel systems: from the fuel filler to the injector. While he made clear that he’s not involved with the process once the fuel gets sprayed into those cylinders, he did have some excellent information for us:

The general consensus is that yes, you can save a few cents per gallon by using the less expensive gasoline in your car, and short term it will likely not have much of an impact. Anti-knock sensors do a pretty good job of minimizing the engine knock.

However, you likely will lose a bit of performance as the calibration in the ECU is based on using premium fuel, so the effective cost per mile may effectively be the same. And when the difference is $0.25/gallon, it’s only a difference of $2.50 per fillup on a car like a Chevy Malibu. Cheaper than a cup of coffee at Starbucks.

Over the long term, it’s not so clear cut. There is some anecdotal evidence that there could be increased wear in engine components, but it has not been quantified – the cost to test this is prohibitive. Think hundreds of cars running 100,000+ miles to get a representative sample to quantify the difference.

So when you add it all up, it’s probably best to stick with the recommended fuel for your vehicle. There just isn’t enough of a benefit to justify the potential added cost.

He also added

I had another thought related to this - to keep your tailpipe emissions in line with what the vehicle is supposed to get, you absolutely should use the recommended fuel. So much calibration work goes into minimizing the tailpipe emissions that changing from premium to regular gasoline will have a negative impact on your vehicles emissions. Probably not enough to make it an issue for your car getting certified, but definitely will make your car less environmentally friendly.

So, here are your takeaways:

Advertisement

• You’re most likely not going to kill your engine in the short term by using a lower grade of fuel, thanks to how well anti-knock sensors do their job

• The money you save, though, may be lost anyway, because the engine is designed to provided optimal efficiency and performance with a specific fuel type. Changing that will likely lower power output can cause it to use more gas, possible making your at-pump savings a wash.

Of course, if the difference between premium and regular keeps growing, the math becomes a bit different. If premium continues to rise faster than regular, using the cheaper gas actually will save you money, at least short-term. However:

Advertisement

• You’ll very likely pollute a bit more. You’re killing Mother Gaia, you monster.

• Long-term effects simply aren’t really known. So there may be a risk of some sort there.

So, if you’re looking for a rule of thumb, I’d say use the fuel you’re supposed to use. Unless you’re really broke, in which case you can cheat every now and again without much worry. It doesn’t seem like it’s that big a deal.

Advertisement

Advertisement

But, overall, if you want to keep the car for a long time, why push it? $5-$8 or so per fill-up isn’t great, but think of it as preventative maintenance for your engine’s long term health.

If it’s a rental car, fuck it, use the cheapest crap you can find, right? That’s how we’ll build up our test fleet of cars that never get the fuel they need, too!