An Initial Review of the Impact of Fuel-Efficient Driving During Long Commutes

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Before I get into what this is about, it’s very important that I make it clear that the results I’m presenting here are not final, by any means. The calculations I’m doing are rough and not refined, at all. Consider this more of a “fuzzy” impression rather than an actual result. But these basic calculations still present some interesting findings.

I’ve been waiting a long time to start this post and, originally, I thought it would only be a single post. However, the mathematics to present very specific findings requires some thought and, simply put, I am not proficient in maths.

Consider this a very high-level observation, perhaps more similar to looking at a city from space, as opposed to viewing a house on a specific street.

My final goal, which is not presented here, is to provide calculations that show the amount of fuel, and money, I’ve saved with the savings based on the price of gas at each refueling. That’s not here. Instead, for this post, I’m only working with grand totals and an average for fuel price. And the values that I’ve presented for a couple of “what if my MPG had been lower” scenarios are based on the number of miles that I drove. A true representation of the number of fill-ups would need to account for the fact that I can’t put more than 17 gallons in my car, so if my MPG dropped, I’d actually have more fill-ups.

So, let’s begin.

After we moved, nearly three years ago, and I started commuting to the same job, over a much longer distance, I began making efforts to improve my driving in order to get better fuel economy. A couple of months into it, I began tracking every refueling, logging the odometer reading, gallons pumped, cost per gallon and some other values. I also trained myself to drive my car more efficiently.

The difference was noticeable. Over time, I achieved a higher MPG on the majority of my drives; it’s not unusual to hit 28 MPG though I’ve actually exceeded 31 MPG. If you’re being skeptical, and you noticed that I didn’t have a control in which I properly logged “normal” driving before this experiment, then I’ll admit that you’re correct but I am working from some previous information. The vehicle itself usually showed that I was getting 23-24 MPG, on average. And this wasn’t the first time that I started logging my MPG; I’ve done this in the past and typically it would fall around 24-26 MPG.

Here’s some information about my vehicle. It’s a 2008 Chevrolet Impala with a 3.5 L, V6 engine, which is rated for 18 MPG city and 29 MPG highway. Max fuel capacity is 17 gallons, though I think there may be an additional half of a gallon in the system, but I stop fueling the first time the pump shuts off. I don’t try to get the extra squeeze or two in. Government and other resources rate it as having a 22 MPG combined rating. User data from sources such as Fuelly, appear to agree. Generally, I’ve taken good care of the vehicle, especially in its earlier years.

I do have a pair of ageing “Fuel Fighter” tires, which supposedly help with fuel economy, and I had a fancier air filter, at one time, that maybe (and I mean maybe) made a slight difference, but otherwise there is nothing else special, or custom, about this vehicle. I use normal-grade gasoline; I’ve only used premium a few times and it didn’t show a difference with my car to merit the additional cost.

There’s a reason I mentioned “long commutes” in the title of this post. It’s difficult to drive more efficiently when driving in-town. The intersections, frequent stops and starts, etc put the car in a different operating condition than when it’s driving down a highway with only few stops or slow-downs in between. The latter one describes the majority of my driving. My round-trip commute is over 100 miles per weekday. In addition, most of my commute occurs on a major road with two lanes for each direction of travel. It’s far easier to drive efficiently when you are less concerned about other drivers, which is the type of driving condition that most of my miles occur on.

I do have a bit of in-town driving and it’s often reflected in my MPG. Some weeks my MPG hovers just above or below 25 MPG. I’ve attained an MPG of slightly over 30 MPG, but it’s uncommon. One fill-up revealed an MPG of 31.59. It’s certainly a statistical anomaly but I do think it’s accurate. I re-checked my numbers and have concluded that a set of random circumstances lead to this MPG, which would be difficult to reproduce, especially since I don’t know what they were.

My highest MPGs, near or slightly above 30, generally occurred in the winter months, when I didn’t need to use the air conditioner.

What does it actually mean to drive more fuel-efficient? For starters, avoiding fast accelerations or anything else that may rev the engine. Taking advantage of coasting (though not to the point that I’m negatively impacting traffic or dropping well below the posted speed limit; absolutely nothing that involves shifting into neutral, etc) and also managing my braking more cautiously. and anticipating the moves of other drivers. For example, if a pack of cars comes flying up, I’ll avoid getting among them and instead let them pass (I’m usually cruising right at the speed limit, so most drivers will pass quickly). Getting close to them would likely mean many break taps and unnecessary accelerations.

Again, this isn’t how one can drive in-town. It’s better to go with the flow of traffic, for the most part, though sometimes it’s possible to do the small things here and there.

It’s worth pointing out that my logged MPG values are often on the high end and, in some cases, higher than the manufacturer’s MPG rating, which is interesting because those are generated under ideal conditions that most consumers will generally not experience in the real-world.

On to the numbers.

Note that I’m purposely not including my individual transactions. Believe it or not, those simple details might reveal more information than I want to include here.

January 2017 – August 2019

Total Miles: 64,199
Times Refueled: 161
Total Gallons: 2,373
Average Cost/Gallon: $2.246
Total Fuel Cost: $5,329.05
Average MPG: 27.36

Those are interesting figures, but the goal of this tracking is to figure out what I’ve saved in fuel cost and gallons. OK, once again I’m going to reiterate that these figures aren’t necessarily accurate, at this stage. These are based on sums and averages; I have yet to calculate the actual savings unique to each refueling.

But this may give us an idea of whether or not I’ll be likely to find a savings, should I actually get down into the details.

I’m going to be a bit conservative in the first set of numbers and base them on an average MPG of 26. That’s above the combined average, and it appears to be on the high-end for most owners of my vehicle, though it’s also not unusual for many and is probably close to what I was getting before I altered my driving habits. Note that for both of these what-if scenarios I’m basing my calculations on the number of miles that I did travel and using the average cost per gallon – again, the real details in these spots may show that I actually saved more, or less, than these figures would imply. Note that the differences for each set are compared to my actual fuel cost and usage, and not each other. If you want to double-check the math then look back to the total number of miles that I drove, above.

26 MPG

Total Gallons: 2,469
Total Cost: $5,545.80
Difference in Gallons: +96.15
Difference in Cost: +$216.71

24 MPG

Total Gallons: 2,674.96
Total Cost: $6,007.96
Difference in Gallons: +301.92
Difference in Cost: +$678.91

For one thing, it’s very clear that improving fuel economy by only a couple of miles/gallon can make a difference, much like turning an air conditioner up a couple of degrees can lower an electric bill.

Some might shrug and suggest that it’s not that much of a savings over the time range, though that’s entirely subjective (and I’d also point out that even the lead-foots are often willing to drive across town to save a few cents on gas) . Consider that difference between my actual MPG, compared to driving at 26 MPG, currently equates to at least a week’s worth of fuel conserved, for my commute. Compared to 24 MPG, that’s at least 3 1/2 weeks of less fuel used for the same number of miles. Of course, once again, that’s based on these generalized figures.

Feel free to point out any errors in the math that I’ve used, if I haven’t already pointed them out myself.

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