Premium gas sounds like it packs more power than regular gas, right? Not exactly. Premium fuel contain no extra energy—it just resists “knock” better (more on that later). For certain rare engines, this can improve fuel economy. So, is it worth the extra cost? That depends on your car. Here’s how to tell if premium gas is actually saving you money or just draining your wallet.

Does premium gas improve mileage over regular?

Sometimes—but only if your engine is designed to take advantage of it. Many turbocharged or high-compression engines adjust timing and boost when running premium, improving efficiency. But for most cars, it’s a waste. Car and Driver found a Ford F-150 EcoBoost gained 0.6 mpg on premium, while a Honda CR-V saw no difference.

Premium gas better withstands pressure. What does this mean? It is less likely to pre combust or “backfire” (creating a knocking noise). For an engine with high compression it’s mandatory. And for a more adjustable engine, it can boost MPG.

Some engines can tweak their performance based on fuel quality, but the gains are honestly small. Progressive explains that premium fuel resists knock, allowing certain engines to run higher compression safely. But if your car is tuned for regular, premium regular gas won’t help your mileage. The best way to find out? Run a test tank and compare your mpg.

Can I mix premium and regular gas?

Yes—if your car can run both, mixing is fine. You’ll end up with an octane level somewhere in the middle. Just don’t mix ethanol-free and ethanol fuel in a vehicle that’s not designed for both. That’s a different issue entirely. When in doubt, check your owner’s manual before blending fuels.

Most modern cars have sensors that adjust for different octane levels, so half a tank of premium won’t cause any problems. But don’t cheap out if your car requires premium—Car and Driver found a BMW M5 lost power when running lower octane fuel. Worse yet, knocking can mess up your engine. If your engine is built for high-octane gas, regular fuel can cause real damage.

Should I ever use premium gas in a car designed for regular?

It probably won’t do much. Premium fuel only helps engines that can adjust for it. Car and Driver tested a Dodge Charger R/T, which recommends mid-grade but can run regular. The extra octane made little difference in performance or fuel economy–adding just 14 horsepower. Progressive confirms: if your car doesn’t require premium, using it probably won’t do anything.

That said, if your car recommends but doesn’t require premium, a test tank might be worth it. If you see a noticeable mpg boost, the extra cost might be justified. But for most drivers, premium regular gas just means paying more for the same results.

The bottom line on premium vs. regular gas

If your car requires premium, use it—running regular could cause serious engine damage. But if the manual recommends premium, test a tank to see if it actually improves mpg enough to matter. If your car is built for regular, don’t bother. You’re just paying extra for no real benefit.