Do you have a 5.7-liter HEMI V8 that recommends you use 89 octane fuel (often called midgrade)? Once you realize that most gas stations have two tanks and “splash blend” premium and regular to get midgrade, you’ll realize just how overpriced 89 octane gasoline is.

Take, for example, Redditor MonkeyMan47 who posted a photo of a gasoline pump to the “Ask Mechanics” forum last year. He asked, “Rather than paying $4.69 for 89 octane fuel would it be the same to fill half a tank each of regular and 93 to save money?”

The response was a resounding “mix away!” But his photo reveals just how overpriced midgrade gas can be. In a station with 87, 89, and 91, the midgrade is truly in the middle and should be priced accordingly. But at this station, the premium is 93. It cost $4.79. So charging $4.69 for 89 octane is highway robbery.

Mixing your own fuel could save you 50 cents a gallon, or more.

Let’s do the math: 20 gallons of premium at $4.79 costs $95.8. But ten gallons of premium plus ten gallons of regular ($3.69) would cost $84.80 ($36.9 + $47.9). That’s an $11 difference for 20 gallons, over 50 cents in a gallon in savings. And you would save even more if you mixed 87 and 91–which still makes 89 octane fuel.

Do note that if your minimum octane rating is 89, you don’t want to dip far below. But a higher octane rating will be safe. So if you mix your own gas, mix conservatively. It’s not a bad idea to aim for 90-octane, so you don’t accidentally run 87 or 88.