Blending gasolines...

Dear Car Talk

Dear Car Talk | Oct 01, 1991

Dear Tom and Ray:

I'm a professor, although not in engineering. I have a '91 Acura Legend that requires 91 octane gasoline. Most gas stations around here stock 86, 87, 89, 93, and 95 octane. One station stocks 89 and 93 at prices 5% less than competitors. If I fill half the tank with 89, and half with 93 at the same time, will I have 91 octane in the tank?

TOM: The answer is yes, you will have 91 octane, Alan. Or certainly close enough so that the car won't know the difference. In fact, if you've been to a Sunoco station in recent years, you've probably seen the same thing being done right at the pump.

RAY: Sunoco stations allow you to choose between a variety of octane settings, and your choice is actually blended right there at the pump--just like the coffee at A & P.

TOM: Sunoco's method is actually a little more precise. They use Taylor Expansion Linear Blending Equations to determine the exact octane of two gasolines being blended. If you were a mathematics professor, I'm sure you'd want to take your programmable calculator down to the self serve pumps and figure that out, too. But for a humanities professor, mixing them is just fine.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One