I saw a thing on the Internet the other day about mpg (miles per gallon) vs mpd (miles per dollar). What the guy did was a gasoline-consumption comparison between different models of vehicles. But the real cost per mile to drive a car should include the cost of the car, the cost of maintenance, insurance, and various other costs. That’s what the government does when it gives mileage allowances to those who use their own vehicles in service of the government. As of 1 February 2007, the allowance was $0.485 for the average car–almost 50 cents a mile–and as of 1 January 2008, almost a year later, the IRS allows $0.505, just a smidgen over a half a buck.

But, if you already own a car, that kind of comparison is more or less meaningless. What you might want to do is figure out how to buy gas when you already have the car. And that’s what I’m about to do.

I once watched the love of my life (she’s gone, now) pump gas at a self-serve. She was buying the 87 octane because it was "cheaper"–about a dime cheaper than the 89 octane and 20 cents cheaper than the 91. I asked her if she ever tested her Ford Aerostar to see if it was cheaper per mile with the 87 octane, 89 octane, or 91 octane. I explained that with my car (a 1995 Honda Civic) I got something like 11 percent better mileage with the 91 than I did with the 89 while the difference in price between the two grades, at the time, was something like 7 percent. I said the comparable difference between the 91 and the 87 was greater. She looked at me as if I were an idiot.

Now, she was bright, perhaps the brightest woman I’ve ever had a relationship with. In fact, she was working toward a degree in mathematics. Yet, my question was lost on her and, not being one to bang my head against a wall (okay, sometimes I do that), I let it go.

Nowadays, I own a 2003 Honda Accord. The price of gas, in the meantime, has risen. But I’ve noticed that the difference in price among gasoline grades has more or less remained a constant 10 cents–the 91 (92 octane here in Oregon) is still about a dime more expensive than the 89 and 20 cents more expensive than the 87. But, with the price of premium hovering in the mid-three-dollar range, the percentage of increase in price is now down around 3 percent but my increase in mileage is around 10 percent. That means that while the cost of gasoline has gone up, the mileage I’m getting from the different grades has remained more or less constant. So, ironically my savings while using the "expensive" gasoline has become considerable.

In fact, for each thousand miles I drive, I’m saving about $11. Besides that, I have to make fewer fueling stops, and I hate stopping for gas unless the attendant is of the female persuasion and cute. On top of that, I notice my car performs a little better

So, I’m suggesting that if you want to save some money when you buy gas, find out how much you spend per mile of driving or, as I did it, how much I save per thousand miles of driving. To do this, let your tank run close to empty, then fill it with one grade. Note the price. Then, when you’re close to empty, again, figure out how many miles you get to the gallon. Now, fill it with the second grade, note the price and, when you fill up, again, also note how many miles you got to the gallon. Repeat this with the third grade. Now you can figure out how much 1,000 miles of driving costs with each grade.

(By the way, with the price of a gallon of gasoline shifting the way it does nowadays, write down the price of each grade each time because when you do your final calculations you want to be comparing the cost if you’d bought one of the other grades that day.)

Say you get:
24.9 with the 87 octane
26.4 with the 89 octane
29.8 with the 92 octane

In the first case, divide 1,000 by the 24.9 and that figures to 40.16 gallons of gas to drive 1,000 miles. If gas is $3.299, it’s costing you $132.39 to drive 1,000 miles on 87 octane.

In the second case, divide 1,000 by 26.4 and that figures to 37.88 gallons which if the 89 octane is a dime more expensive means it costs $128.75 to drive 1,000 miles.

In the last example, divide 1,000 by 29.8 and that figures out to be 33.56 gallons of gas and, if the dime difference holds true, it costs $117.42 to drive 1,000 mile on the 92 octane.

You can see the savings per thousand miles. Estimate how many miles you drive annually and you can figure out roughly what it costs to drive and calculate your yearly savings.

But, once again, as I said, when you calculate the savings, use the same-day price from the three grades. E.g., if the price of all three grades have moved ups 20 cents, do the calculations with the latest figures so you can do an accurate comparison.