Buying premium versus regular gas:
I just got off the phone with my mechanic (Charles Matthews of Cornerstone Auto (602) 277-1549), and he clarified a few things for me. He said that mostly the improved MPG came in older gas-guzzler cars, and most cars today are not expected to improve MPG from upgrading the gas. However, pinging and knocking, if your car does that, may be fixed. And, obviously, some higher-performance cars require it. Otherwise, over the long haul, higher grades of gas have a tendency to keep the engine running better, meaning that MPG over a long time will not fall off, because the machine is simply "better lubricated," so to speak. These benefits come more from the additives than they do from the octane rating, much as if you were using an additional gas additive. That being the case, the brand of gas you buy makes a big difference, because not all gas brands have the same composition. He recommended you stay with good quality "top tier" gas brands, such as the ones listed on this page. Chevron puts additives in all grades of its gas that makes them all beneficial in this way.
I asked him if the MPG could ever be improved, and he said possibly, depending on the car. So, my advice is to try it and see. Because the percentage difference between the grades of gas is so small whereas the base price of regular unleaded is so high, there is a definite savings to be had from buying the premium gas IF MPG goes up. The short version is that the improvement in MPG needs to exceed the base price of gas times your current MPG divided by the price difference. So, for today, $4.10 * Current MPG/$0.20 will give you how much impovement you need to see to break even. For cars 20 MPG and under, it's 1 MPG, for cars 20-40 MPG, it's 2 MPG. When I try it this week, I'll let you know. However, I've already had some feedback from others.
Emjay wrote me, "I listened to your Friday night show about fuel economy right before I filled my tank. My car normally gets an average of 20.7 mpg, and I wanted to see if I could do better. I went ahead and used the higher grade fuel (.10 more per gallon), stopped doing jackrabbit take-offs and made a conscious effort to slow down and keep my speed steady. Over the weekend I drove both on the freeways and surface streets, and as of this morning my car is averaging 26.1 mpg! That means that on my tank of gas I will be able to drive 417.60 miles instead of the usual 331.20 – quite a difference. Thanks for your show. It’s always interesting, always informative and always makes me think. "
Of course I can't know whether her results are from better driving or from the mid-range gas, but there you go. One caller to the show said he sets his cruise (another MPG saver) on 60 and has increased his per-tank mileage from 300 to 350. Bill, my producer, says that going from regular to plus (mid-grade) made his Civic get 41.8 instead of 40, but he intends to try again to be sure. That would be saving him money, by the way, because he's only spending $0.10 more per gallon, not going to the super gas at +$0.20.
Which car's MPG matters more?
The freakiest thing we learned last week is that, quite contrary to your intuition, improving the gas mileage of a low-performing car has a disproportionate benefit to your finances compared with improving the gas mileage of a better-performing car. This is because the key number is cost per mile, which is simply the price of gas divided by your MPG. When a lower MPG (10-15 base, say) car improves even 1-2 MPG, it's going to save you more money than improving your higher MPG car (25-30 base, say).
Here are some of the numbers I crunched and my conclusions based on $4.15/gallon gas.
12-13 goes from $0.346 per mile to $0.319 per mile. 2.7 cents saving per mile
33-42 goes from $0.126 per mile to $0.099 per mile. 2.7 cents saving per mile
10-12 goes from $0.415 per mile to $0.346 per mile. 6.9 cents saving per mile
20-30 (you'd expect 24) goes from $0.208 per mile to $0.138 per mile. 6.9 cents saving per mile
30-60 (you'd expect 36) goes from $0.138 per mile to $0.069 per mile. 6.9 cents saving per mile
So, if you have two cars that you drive equally, you should focus on the one that gets the worst gas mileage. Even if you have a daily drive that you drive more, You should probably upgrade the other one, because it winds up saving you more, most likely, even if the improvement is modest.
15 mpg to 20 mpg = $0.277/mile to $0.208/mile = 6.9 cents per mile which is $69 per 1,000 miles.
To get the same savings on your daily driver that already gets 30 mpg ($0.138/mile):
Driving 1:1, you’d have to get 60.1 mpg ($0.069/mile)($0.069/mile dif)
Driving 2:1, you’d have to get 40.1 mpg ($0.104/mile)($0.035/mile dif)
Driving 3:1, you’d have to get 36.1 mpg ($0.115/mile)($0.023/mile dif)
Driving 4:1, you’d have to get 34.4 mpg ($0.121/mile)($0.017/mile dif)
So, if you're trying to decide which of your two cars to upgrade based on fuel costs, it's probably a better bet to upgrade the SUV/Minivan the wife drives around town than the economy or standard car the husband commutes to work in.
Feel free to post your own results so that others can benefit from the information or email them to me.
Bill just found this today (6/24/8). It's hilarious...and scary. Hypermilers by USA Today. Here's a thread I found with a bunch of suggestions at CleanMPG.com
More updates (7/3/08). Bill just got the first data back on his use of mid-grade gasoline. His 2008 Civic had previously gotten 41.8 MPG, and his first trial yielded 44.2 MPG. He intends to try premium next and also go back to regular afterward to see how it all compares. He also notes that the increase may be due to some of the hypermiling techniques, so we'll know more when he goes back to regular. But so far, great success! I just filled up with premium last week, but I've gone a whole week without even being near needing to refill so far. I'll report the numbers when I get them.
Here's two more links of interest.
Does rolling down windows vs AC save MPG?
10 Ways to Save Money on Gas
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