Should you fill up with regular or mid-grade gasoline?

by Rocket Finance

Last week I mentioned that we were leaving on a cross-country road trip and that I was going to conduct an experiment to see how much our gas mileage improved with mid-grade gasoline. Deamiter was quick to point out many potential problems with my experiment. He was right and the results of my test are not conclusive. There were too many variables.

My original plan was to fill up with mid-grade gasoline during our 1,000 mile journey to Wisconsin, drive around for the first three or so days of our visit and then start purchasing regular unleaded about half-way through. We would use each grade for approximately 1,500 miles and for a similar ratio of town to highway driving.

The first part of the plan worked great. In fact, it was better than great. A supply and demand fluke caused most of the gas stations in Nebraska, Iowa and Illinois to price mid-grade unleaded below the price of regular unleaded. We paid up to $.10 cheaper for mid-grade all across the United States. I meticulously recorded the number of gallons and miles and figured our miles-per-gallon at every stop. However, I also started to realize that the test was not very scientific, here are some of the reasons:

  • We used several different brands of fuel. There really aren’t too many gas chains that are consistent from CO to WI. Furthermore, when traveling with three kids, five and below, you can’t always pick where and when to stop.
  • I don’t think that all gas station pumps “click-off” at the same point. I suspect that the difference can be as much as a quarter of a gallon. Enough to throw off MPG calculations.
  • We did not alway fill up with the same amount. Sometimes we had to stop after two hours and once we went four hours without a stop! We filled up every time.
  • Co to WI is mostly downhill. WI to CO is mostly uphill. Plus we went a slightly different way on the way back.
  • Van weight varied quite a bit. Of course, we unloaded after arriving, thus driving around with an empty van for a week. On our trip back, we acquired more stuff and I am sure that I gained 5 lbs after eating my mother’s food for a week.
  • Our oil was significantly newer and cleaner at the beginning of the trip than at the end of the trip.
  • Unfortunately, mid-grade gasoline was still cheaper in Iowa on our return trip, so while we used mostly regular unleaded for the last half of our trip, At least one tank was mid-grade. I couldn’t bring myself to pay more for gasoline just for a blog post.
  • Regular unleaded is 85 octane in Colorado, in the rest of the states it is 87 octane. Mid-grade in CO is 87 octane and 89 octane almost everywhere else.

I had hoped to post most of the numbers here so all of you could see the variations in MPG. In all of my calculations, I figured everything out to the hundredths place – 15.67 gallons, 21.95 MPG, etc. I was very proud of all of my math. The problem was that I kept the record of our fill-ups on a 3×5 card. I love 3×5 cards and always have one or two around. On the day that we were loading up the car to go home, my dear wife decided to clean out the area around the cup holders and dash. I probably had a half-dozen cards with various scratchings – phone numbers, practice plans, post ideas, etc. Most of the cards did not contain anything important, except for that one card with all the numbers. However, all of them ended up in the trash.

Mrs. Rocket felt bad and tried to make me feel better by offering to call my parents, get them to dig the card out of the trash and read off the numbers to us over the phone. I did not think this was such a great suggestion. She also said, “I think your posts have too many numbers anyway”.

I realized that this was the final straw for my poorly executed experiment.

In spite of all the trouble, I think that I was able to generate a working hypothesis. I believe that mid-grade gasoline improves our gas mileage by two to three MPG for our 2005 Dodge Grand Caravan. I say this without the rigor of empirical data, just a sense of how things were trending with the data I had available and taking into account all of the factors. When regular unleaded is $4.00 a gallon, our cost per mile is $.21. If we conservatively assume that we get two more miles per gallon with mid-grade gasoline, we save money by purchasing the mid-grade as long as it is cheaper than $4.33.

I plan to buy mid-grade unleaded when the price difference is less than 8% of the price of regular unleaded. If you need help in how to figure out percents, send me a message.

I think that I really need a MPG Meter.

  1. 4 Responses to “Should you fill up with regular or mid-grade gasoline?”

  2. By Chris Eaker on Jul 22, 2008 | Reply

    You are right that there were too many variables and varied conditions to really make an accurate conclusion. There was one time when I put premium in my car for some reason I can’t remember and my mileage was quite a bit higher. I can’t remember the actual numbers, but it was almost enough to make me change for good. Overall, it’s probably a wash with the extra price for the higher grade fuel. But I also wanted to point out that the different brands of gas don’t make much difference. The gas you buy at a Chevron station could very well have been refined at an Exxon refinery and the gas you buy at Exxon could have been refined at a Shell refinery. It all goes into one pipeline and is delivered to various places around the country. However, the different companies do add their own special additives to the gas when it arrives at the final destination, but I don’t think they are much different.

  3. By rocketc on Jul 22, 2008 | Reply

    I wish I knew for sure. I’m sure there is a price at which it makes sense to buy the higher grade stuff.

  4. By Lynda on Jul 23, 2008 | Reply

    The price of mid-grade gas in Iowa, Illinois and Nebraska is subsidize by use of up to 10% ethanol, which can lower the actually energy produced. I should by unleaded but actully buy mid-grade because it is cheaper..

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