Dear Car Talk:

I have a 2016 Toyota Avalon. The manual says that I should use 89-octane gasoline. I like to buy gas at my local Costco store, where the price is always lower than at the neighboring gas stations. Unfortunately, my Costco offers only 87- and 93-octane gas. I'm too cheap to buy the premium gas, but I don't want to hurt my car by using the cheap stuff. So every time I fill up, I buy a half-tank of each octane rating, figuring that it will average out to 90 octane. My wife says it doesn't work that way. Help me prove her wrong.

-- Dave

That's exactly how it works, Dave. In fact, gasolines often are blended like that right there at the pump. So the gas station will have a huge, underground tank of 87 and a huge, underground tank of 93, and will mix the octanes in between by blending those two tanks in different proportions, depending on what grade the customer selects.

So you can continue to make your own home brew. But before you do that, check with Toyota. When we looked up the fuel requirements for 2016 Avalon, it says that the Avalon requires regular unleaded gasoline. That's normally 87. So if all your car needs is 87, you're not only ticking off your wife, but you're throwing away your time and money, too.

Any one of those negative factors might, in itself, be reason enough to change your behavior. But with all of them against you? You might want to reconsider your gas-station bartending and stick with the 87, Dave.

And by the way, to reinforce the point, AAA just did a study that concluded what we've been saying for decades: that if your car doesn't require premium gasoline, it's a complete waste of money. You get no benefits from it -- no increased power, no better fuel economy, no cleaner engine, no nothing.

Buying premium fuel when your car requires only regular is not like paying more for a nicer hotel room; it's more like paying more for a bigger shoe size. It won't help you -- it'll just cost you money.