For years, I’ve always wondered about the three grades of gas offered at the gas station. While I know that they recommend the higher grades for certain vehicles — I always use the mid-grade for my Land Rover, and I’m supposed to use the high grade for my scooters — you have to wonder if there is really any difference between the grades.
Can you get better performance (better gas mileage is the most measurable thing) from using higher grades? More importantly, can you get enough better performance to make it worth buying?
According to this Time Magazine article, a test by Car and Driver magazine says “No”.
However, that’s not exactly what they said. Here is the actual article, from 2001. Summarized, what it says is that cars which are intended to run on regular unleaded get very little or no added horsepower and speed from a higher octane, cars which are intended to run on premium fuel experience a drop in horsepower and other performance from a lower octane, and carbon buildup over time means cars designed for lower octane fuels may need to upgrade due to carbon build up and the like. (One thing I note is that they used only low and high grades — 87 and 91 octane — not the midgrade 89 octane which is also an option.
Before I saw this Time article, though, I had decided to do my own test. I had noticed that while gasoline prices have climbed — I remember that I paid $1.99 a gallon for low grade on Labor Day Weekend in 2001, and that was the lowest in months, vs. $4.15 a gallon yesterday for high grade — the price difference between the grades has stayed the same, about 10 cents a gallon. Which means that as a percentage of the price per gallon, premium grade gasoline has effectively become cheaper — it was a 5% markup in 2000, but only about 2% in 2011 — and that means that the viability of any performance (mileage) increase is only half as much to make it worth while.
So let’s look at my Land Rover. I get between 16 and 19 miles per gallon, depending mostly on city vs. highway driving, although factors like weight in the rear end also factor (if I add a 50 pound piece of equipment, my mileage on a highway trip increases by about 1 mpg). For the sake of the argument, let’s call it 18 mpg.
I have roughly a 15 gallon tank. At $4.00 a gallon for 89 octane (it’s a little more than that now, but round numbers work the math easier), that’s a $60 fill up; at $4.10 a gallon for 91 octane, that’s $61.50. Yup, just $1.50 difference, less than the price of a drip coffee at Starbucks, less than half a gallon of gas; put up against a $60 fill up, that’s almost insignificant. More to the point for doing math, it’s a 2.5% change in the price.
So back to the mileage. If I pay 2.5% more for the gas, then I need to get a 2.5% increase in my gas mileage to compensate. (I’m not sure that’s completely right. With the way percentages work, it’s more likely that I actually need something like a 2.4% mileage increase, but the difference there is not going to be enough to alter the test results any more than my use of “round numbers” will. If someone wants to work out the numbers more exactly, feel free.) On an 18 mpg average, that means 18.45 mpg — increase just 1/2 mile per gallon and I will have “won”.
What did my tests show? I shifted to premium octane for several fill ups, including a road trip to Portland (about 180 miles each way). My highway mpg increased to over 20, and my city average was over 18 (city would normally be in the 16–17 range); calculated out, I got an increase of between 1.2 and 1.7 mpg over the course of the tests.
That is, I got 2–3 times as much increase in gas mileage as I needed to make the increase in cost worthwhile. Which means I actually save about $2–3 per fill up by not needing to do them as often.
Let me note a couple things where my tests differ from those of Car and Driver:
- I’m driving a 2005 vehicle. Technology changes may have improved the performance differences.
- Car and Driver used expert drivers, test tracks, multiple vehicles, and more controlled circumstances, while my test was more informal, real world, and isolated to a single vehicle.
- I tested miles per gallon, while they tested horsepower and speed.
- I used a change from 89 to 91 rather than from 87 to 91, on a vehicle aimed (I think) at the midrange to start with.
- The change in gas prices in the past 11 years has magnified the value of any performance benefits gained, which changes the impact of the results.
- My Land Rover gets relatively low miles per gallon. Cars which already get higher mileage need to have a bigger value increase to compensate. (A car which gets 40 mpg has to break, hmm, 41 mpg. Ooooo…)
- Time Magazine relied on an 11 year old article, but didn’t identify that and the limits that go with it in their article. Shame on them. (Some education for when you read something referencing other sources: check what the original said rather than trusting a summary of it, since that might reflect their own biases. That’s why I’ve provided links to both stories.)
But if you are pained by the cost of a fill up these days and if you are curious enough to track your gas mileage for a month or so, I think you should run the same informal tests for yourself. Get a mileage baseline for both mostly city and mostly highway driving for a few fill ups at the grade you currently use, and then switch to one grade up and track how things change. Worst case, you’ll see no change of note and you’ll be out the cost of a few cups of coffee over the course of a month. But I think there’s a good chance you’ll also see enough of a difference to consider making the switch completely.