Saving money buying gasoline: mpg vs mpd
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.
February 9th, 2008 at 2:33 pm
Great article. You’ve actually rejuvenated my want to put the Super Unleaded in my car. I drive around 30K miles for business per year. Your readers might find this gadget I use interesting. It’s called a mileage logger and can be found at http://www.mileagelogger.com. It’s a GPS gadget that logs my business mileage for me. I’d be hopeless come tax time without it. Thanks.
February 11th, 2008 at 4:41 am
Thanks, John! I always wondered how you had figured this out for the Honda-mobile. I’m off to do some math.
February 11th, 2008 at 8:41 am
Hi John! Interesting post, but of course now I have a question for you. I always used to buy premium gas for my battered little used bright red VW Golfs, assuming that it was better for the cars. But when I bought my most recent battered little used bright red VW Golf, there was a warning posted right at the gas tank saying to use 87 octane gas. So I’ve done that with this car, but have always wondered why. Any ideas?
February 17th, 2008 at 1:28 pm
Great Article, John. One of the most intelligent, analytical blog I have read for a while. I am planning to switch to 92 from 87 on my 2003 Accord EX. This is exactly what I am planning to do. I want to check out the MPG with 92. I think my car gives a better ride and less rumbling with 92.
Thanks,
Amit.
February 24th, 2008 at 8:09 pm
John: Good logic, and something not many people think about. Mostly our focus has been on saving a few cents per gallon here and there by buying the less costly fuel, or using some fuel patronage card at a particular station. For several reasons, this is not always the best approach.
When I was a kid, we used to have gas wars where 22 cents a gallon was outdone by the fellow across town or just across the street selling it for 19 cents a gallon. My perspective has always been to take the difference in price, multiply that by a tankfull and see how much more fuel you can buy with the savings. That should tell you how worthwhile it is to hike around town to obtain the lower cost fuel.
Let’s do a comparison. A 3 cent savings on a 20 gallon tank of fuel brings us 60 cents in savings. In the 1960s, that meant another three gallons of fuel, something worthwhile for a 25 mpg vehicle since that would be result in another 75 miles of travel. Today, that means less than a quart of fuel. For a 25 mpg vehicle, that is another 5 miles of travel. If you have to go to the far side of even a small town to get that savings, you would consume more in cost of fuel than you would realize in fuel cost savings.
My approach to savings today centers on buying in bulk at the lowest prices during the winter (last season $1.95 per gallon, this season $2.91 per gallon), and using it during the summer when the fuel prices are considerably higher than the bulk purchase price. I plan to couple that approach with your advice, and perhaps next winter I won’t be buying 500 gallons of midgrade, but perhaps premium.
Thanks for your ideas.
Clair
March 1st, 2008 at 8:36 pm
Gasoline with a higher octane rating reduces predetonation (knocking) in high compression engines. In other words it makes the gasoline less explosive; This is so the pressure of the combustion chamber won’t cause it to explode before ignition by the spark plug. Unless you have a high performance engine designed for it high octane gas has no effect. Even then it doesn’t make the car run better or increase mpg, it only keeps the engine from knocking. If your car manufacturer calls for 87 you will get no benefit from 91 or 93.
March 2nd, 2008 at 8:43 pm
It would seem as though your comment makes sense: That is if your car is supposed to run on 87 or 89 octane, using a higher octane won’t improve your mileage. But my 95 Honda Civiv was supposed to run on either the 87 or 89 and I got my best mileage and cost per mile with the 91 and 92. I can’t remember what my current Accord is “supposed” to run on. But I get my best mileage and cost per mile on 91 and 92. I’ll take empirical evidence over theory, any day.
March 3rd, 2008 at 5:01 pm
I can’t argue with real world results but I am quite sure your mpg gain isn’t coming from higher octane.
Years ago premium gas was better due to other additives. I thought that in modern gasoline there was no difference other than the higher octane. I could be wrong, maybe there is a difference.
You’ve made me curious now so I’ll try premium for a couple of tanks. If I’m wrong I’ll have to apologize to you and my wife. I convinced her several years ago to use regular.
March 30th, 2008 at 12:01 pm
In the old, carbureted cars, high octane would have no effect on most cars. I did once have a toyota that had an adjustment on the distributor. You timed the car in the middle of the range, then turned the dial to just under the point where you would hear pinging. With that car, I got better gas mileage with higher octane, but not enough to make up for the additional cost.
Today’s cars have knock sensors, O2 sensors etc to continuously adjust the timing and mix for the fuel and conditions. Yes, you can get much better mileage on some cars with high test. Some cars will actually get worse. You have to try it to see.
Just make sure there are not too many variables. I have one motorcycle that gets much better mileage with high test than mid, worst with low, BUT I use low test when it is cold, but have to use mid grade when it is warm, and high test when it is hot to prevent pinging, so I do not know if there is a difference in MPG between the grades, or if it is just the conditions. The differences would be enough to justify higher grades, about 5% between each grade.
May 26th, 2008 at 5:49 pm
I ran two tanks in my Toyota pickup with no change in mpg. However, my wife’s Honda did get better mileage but only from certain gas stations. She found that Sam’s Club high octane resulted in better mpg and saved about one cent per mile. When she bought at other stations sometimes her mileage was worse.