Dear Car Talk:

Where I live, in the People’s Republic of California, gas costs much more than anywhere else. For instance, I recently saw 87 octane for $3.60 a gallon, 89 octane for $3.80 a gallon, and premium 91 octane at $3.90 a gallon. My wife’s older Lexus LS400 runs nicely on 89 octane.

Here’s my question: If I buy 89 octane, it costs me $3.80 a gallon. What if I put in half a tank of 87 octane, and half a tank of 91 octane? Would I get 89 octane at 5 cents less per gallon? Inquiring minds would like to know. — Tom

Yes. You’ve broken the code, Tom. But now that the oil companies know who you are, prepare to see your face on a wanted poster in every Exxon station bathroom.

This is, in fact, exactly how most gas pumps deliver 89 octane fuel. When you choose 87 octane, it gives you fuel from the 87-octane storage tank below the pumps. When you choose 91 octane, you get fuel from the 91-octane tank.

But when you choose 89 octane, the gas pump literally just blends the two fuels as it sends it into your gas tank, giving you half of each, which gives you 89 octane.

So you can do exactly the same thing yourself, and save 5 cents a gallon. The only thing it costs you is your time. But we can calculate that, too.

Let’s say your tank takes 20 gallons of fuel. You start with the 91 octane and you add 10 gallons. Then you stop, turn off the pump, and start again.

Of course, now you have to reenter your credit card, enter your ZIP code, submit your fourth-grade report card, choose the 87 octane, and wait for the pump to turn on again.

Let’s say that adds an extra three minutes to your refueling time. With 5 cents off every one of your 20 gallons of fuel, you save a total of a dollar. A dollar every three minutes is the equivalent of $20 an hour.

So if your time is worth less than $20 an hour, you’re in the black, Tom. Just don’t forget to factor in the dollar in time you just wasted reading this answer, too.