Basics of Fuel Economy

Now you might wonder what purpose a post on gasoline fuel economy has on an electric car blog and, yeah, you’re right. I think it’s good to establish what a normal “baseline” cost with a gas car is to later compare it to an electric car. Plus I did a lot of work in the past and I don’t want to waste it!

Whenever you ask someone how good their car is on gas, they usually reply with something like “Well, $20 lasts me about a week, so I don’t think it’s too bad.” Obviously that reply is lacking and most people seem to have no idea how their car is in regards to its fuel efficiency and how much money they dump into a gas tank week after week and month after month. People just don’t keep good records. What’s even stranger is that people don’t seem to want to know either! It’s a rather simple thing to measure as well. I’m fairly frugal and I like to know where my money is going. And I like numbers and data.

The basic measurement of fuel economy is a figure called “miles per gallon.” It is what it sounds like: it’s how many miles your car can drive off of every gallon of gas it burns. This can (usually) range anywhere from 50 with the modern hybrids to like 10 with your damn Hummers out there.

Measuring a car’s miles per gallon (mpg) is rather easy: you calculate how many miles you drive off a certain amount of fuel used. In practice this is tricky as cars don’t have accurate fuel gauges. My Cavalier had a 15 gallon tank but the “half tank” mark of the fuel gauge seemed be closer to 5 gallons than 7.5. By calculating mpg you can’t just eyeball the gauge. What is usually done is to top your tank off, note the mileage, and drive until whenever. You then top your tank off again and note how far you went since the last fill. The miles you drove divided by how many gallons it took to fill your tank back up is your miles per gallon. And this number is a very hard, concrete, hard-to-argue with number. When my mom found out her car only did about 15 mpgs she was shocked, but that’s the truth of it. She was spending that much in gas and she didn’t even know it. You can’t lie with numbers.

I recommend anyone with a gas car try this over a month or two. You might really be shocked with how crummy your car really is efficiency-wise. And how crummy you are as a driver!

Here’s my data from over 4 years.



I think it’s pretty self-explanatory. You have the plot of miles per gallon based on the week of the year. As you can see each chart has a very sinusoidal pattern to it with peaks corresponding to mid-summer and the troughs corresponding with late January. It probably doesn’t take a scientist to see those as seasonal variations, most likely due to temperature variances; July is the usually the hottest month and January is usually the coldest month.

This isn’t a coincidence as the internal combustion engine suffers many pitfalls during cold weather. Oil is thicker, along with transmission fluid, and the engine takes longer to reach operating temperature. The coolant is also frigid People usually have the defrosters and the heat on which puts pressure on the alternator and therefore the engine. Energy has to come from somewhere and in a gas car it all comes from gasoline. Tires also have a greater rolling resistance in the wintertime. Cars simply use more gas in the winter.

How much more? Well, you can see that. The 1.9 Liter Saturn ranges from 35 mpg in the summer to 25 mpg in the winter: a loss of 30%! The Cavalier fares even worse with a reduction from about 33 mpg in summer to a 37% loss down to 21 mpgs. (As a disclaimer I was running E85 in the Cavalier but also adjusted the math to account for the loss in energy density of the fuel. I might explain that later.) You might want to consider a 25-30% reduction normal, which is still a big number.

One more point about fuel mileage: it doesn’t take into effect the cost of the fuel. This is important as we will later see with electric cars. If your car has bad gas mileage but you can buy cheap $2.00 per gallon gas and you compare it to an efficient car that uses $4.00 per gallon gas you won’t have any meaningful answer. The key here is to take your car’s mpg value and divide it by the cost of fuel per gallon. This gives you the miles you can drive off a single dollar of gasoline. Basically if you want to give someone a car ride you know how much to charge him, precisely, per mile you drive. It’s a useful number.

Using some rough numbers from the Saturn, and considering gas being $3/gal, you get a range of 11.6 miles per dollar, or more usefully (the inverse) of $0.09/mile. So for every mile you drive you expend 9 cents worth of gasoline. Viewed differently, each dollar of gas buys you 11.6 miles of driving distance. That will add up quickly huh? If you drive 10 miles every day for a week you’d spend $6.30 per week. Which amounts to about $25 a month, an average cost of fuel for someone with a car as good as the Saturn! That is also if you only drive 70 miles a week.

Most cars are much worse than the Saturn SL2. If you consider a car that does 15 mpg (a lead-footed driver with a V6 or a V8, or a typical pickup truck) at a cost of $3/gal you end up with cost of $0.20 per mile or 5 miles per dollar of fuel. Consider this person driving 1,000 miles in a month: they would spend $200 on gas alone. Cars don’t seem to use a ton of gas but they’re used daily and frequently and these costs add up. Most people don’t seem to realize how much a gas car actually costs them because they toss money into the gas tank periodically throughout the month and there never is a single “up front” purchase. But by keeping good records and data you get a very decent picture of exactly how expensive a gas car is to operate and it’s expensive.

When you see how much an electric car costs on a per mile basis you will probably be surprised. It makes gas cars look terrible.

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