Miles Per Gallon is a Silly Way to Measure Fuel Efficiency

There have been people on other bulletin boards who have complained about their Prius getting fewer miles per gallon than they thought. One complained that he had been getting 50 MPG and was suddenly only getting 45 MPG. This is a difference of ten percent, but if you look deeper, you’ll see it’s not as bad as you think. Let’s say his daily commute is 100 miles. At 50 mpg, he uses two gallons of gas. At 45 mpg, he uses 2.22 gallons, which means even at two bucks a gallon, he spends only about 44 cents extra for his gas.
Let’s say however, that you drive a Hummer H2. You normally get 10mpg, and suddenly begin to get 9 mpg. You shrug, since it’s only 1mpg. But if you have the same commute, you would use a whopping ten gallons regularly, and eleven gallons when you lose the ten percent. It would cost you two more dollars a day to drive your commute.
For one week, the Prius owner would pay twenty bucks for gas, the hummer owner one hundred. You’d think if gas mileage was presented this way, then more people would look closer at their gas mileage, and maybe prius owners wouldn’t obsess so much over their gas mileage.

Note: I read an article in Car and Driver making this argument, but for some reason it’s not on their website.

I think mpg is not a silly way to gauge ‘efficiency’. There are two issues in your poast: First, the absolute mileage; and second, the percentage of varying mileage.

Look at it this way: If you get 50 mpg and have a 100 mile commute, then you’re burning 10 gallons of fuel per week. At $2.00/gallon, that’s 20 bucks. If you get 10 mpg, then you’re burning 50 gallons per week. That’s $100/week. So a car that gets 50 mpg saves you $80/week or $320/month.

Absolute mpg is a good measure. If you find your mpg goes down by a certain amount, then it depends on the vehicle how bad that is.

(This is a pre-coffee post.)

The guy with the huge MPG drop might want to have his tires checked.

Yes, determining MPG savings does depend on a little financial math, but I submit that those who won’t be bothered with the math shouldn’t own a car.

Besides, those who buy the SUVs and the Jeeps are mostly unconcerned with the gas guzzling aspect of these vehicles.

It’s the only way of measuring fuel efficiency that is consistant between all vehicles and all drivers.

A silly way of measuring fuel efficiency is people who talk about how much it costs them to fill up their tank. It just so happens that larger tanks tend to be on larger vehicles that get lower gas milage, but it’s still a misleading measurement.

I understand Eruopeans use liters per 100km. Perhaps gallons per 100 miles would work in the US. It’s proportional to cost, for a given driving route. If you drive 1000 miles a week and your gas mileage is 3 gal/100mi, you know you need 30 gallons. It’s somewhat easier than calculating (1000 mi / 33mpg = 30 gal).

What other possible way to measure “fuel efficiency” is there than in units of distance per units of volume? How else could one compare different vehicles for fuel efficiency if not for standard measurements that do not depend on what vehicle you are driving? If a person is so dense that they can’t calcuate percentages after guaging differences in km/L (or whatever), the problem is in their pea brain, not in the way they measure things.

Well, I think what the OP is saying is not that it’s really a bad measurement, I think he’s saying that a different expression of the same measurement would make the point hit home more. If you’re shopping for a car, and the sticker says, “This car uses about 33 gallons of gas every 1000 miles, which works out to about $66 at $2 a gallon,” more people will understand that than if it said 15 mpg.

Yes, but then they’d have to explain the finance terms in English as well and no one would end up buying car.

I think what we’re seeing is that the type of people who but Priuses are the type who obsess over gas mileage, and the type of people who buy Hummers are not the type who obsess over gas mileage. I don’t think that will change. You would just find the Prius owners bemoaining the extra 44 cents a day, while the Hummer owners pay no attention at all to the $2 extra they are paying.

Personally, I measure mileage in dollars per month. It has stayed pretty much constant over the years, for some reason.

I was at an Acura dealer in Illinois recently. All of the new car stickers included “average fuel cost per month” figures in addition to the MPG estimates. I had assumed that this was a state or national labeling requirement. Is it a requirement or did I assume incorrectly?

That’s interesting. Do you correct for inflation? Does it stay constant as a percentage of total outlay?

It stays constant as to dollar amount. I have only been keeping track for about 5 years, however. It is probably mostly coincidence.

Actually I told a lie - I don’t track it by month, I track it by 2-week interval, because I get paid every 2 weeks, so that tells me how much of my paycheque goes for fuel ($55). I work it out using a 12-month moving average, so that will smooth out any changes, but I haven’t had to change the budgeted amount by even a penny for the last few years. I just seem to drive less whenever the price of gas goes up. Also, I bought a more-efficient car last year, coincidentally at about the same time that gas prices made a substantial jump.

Really? You think people are that stupid? I have yet to meet the person who is incapable of understanding the concept of “per”. If I go into a coffee store, for example, I never see people who are unable to grasp that if it costs $10 per pound, that 2 pounds will cost $20.

But some units are easier to grasp intuitively. How would you fell if your tachometer displayed rotational period in milliseconds? The concept is easy to understand, but it would be harder to interpret the readout.

Wouldn’t bother me. It’s really pretty arbitrary anyway. There’s really nothing special about rpm; the relationship between rpm and velocity depends on the gear ratio anyway. It wouldn’t matter what units the tach used, so long as it still has a big red mark at the point where engine damage will occur. It’s all a matter of convention.

Maybe I’m overstating my case, though. I don’t think it’s a bad idea to indicate average fuel cost per month, because I’m sure most people haven’t taken the time to calculate that, and it could have an impact on their decision. I just don’t think people are so dense that they don’t at least understand the concept that fewer miles per gallon means it costs more money to operate the vehicle. I think pretty much everyone gets that.

“My car gets five rods to the hogshead, and that’s the way I like it!”—Grandpa Simpson (#1)

I cannot think of another way to measure fuel efficiency. Doing the dollars per month thing kinda works as long as the price of fuel doesn’t move much and the amount of driving stays the same.

I do believe that some magazines (Consumer Reports? Car and Driver?) do estimate the average fuel cost for different model cars.


#1. I find there is a Simpsons quote for everything