miles per gallon vs gallons per 100 miles

[Portland Man]

I don't think MPG is a very effective way to convey fuel economy, I've heard it suggested before to use gallons/100 miles, in a lot of ways it makes more sense.

As a for instance 12mpg converts to 8.3 gallons/100 miles and 30mpg converts to 3.3 gallons/100 miles.

This makes it easier to compare fuel costs between vehicles.  How much do you save going from 12 mpg to 30 mpg?  If you convert it to gallons/100 miles, you can see that you save 5 gallons per 100 miles.  Multiply that by the number of 100 miles you expect to drive and the cost of fuel and bam - fuel savings.


The main reason I'm thinking about this is to consider the law of diminishing returns.

For instance - going from a light duty pick-up at 18 mpg to an avg passenger car of 25 mpg saves you 1.6 gallons per 100 miles.  If you drive 10k miles per year and gas costs $4/gal it's clear that you will save $620/year.

Now, if you've already got a 25 mpg car, and you are on the fence about stepping up to the bare minimum mustachian mileage of 35 mpg, you'll see a savings of 1.1 gallons/100 miles.  That's $457/ year.  Is it worth it?  I don't know, but you obviously shouldn't spend much money on the upgrade expecting to recoup costs quickly. 



Quick and dirty chart:
            @$4/gal
mpg      gal/100mi      $/100mi
12      8.3      $33.33
15      6.7      $26.67
18      5.6      $22.22
20      5.0      $20.00
25      4.0      $16.00
30      3.3      $13.33
35      2.9      $11.43
40      2.5      $10.00

[Bakari]

http://ecomodder.com/forum/showthread.php/why-gallons-per-mile-l-100-km-makes-3192.html

[RussellMania]

You are right that gallons/100 miles is much more clear in terms of clarity of fuel use.

My only problem with it is probably a semantic argument more than anything. People often talk about "fuel efficiency". In the standard definition, efficiency = "what you get" / "what you give". When applied to cars, "what you get" = miles, and "what you give" = gallons, hence miles/gallon. The problem is that it is nonlinear (ie, doubling MPG does not halve fuel use), as you have pointed out, which is extremely confusing for many people.

So, if we stop calling cars with low g/100 m "fuel efficient" and start calling them "low fuel use", or something similar, I like the idea.

[Jamesqf]

I disagree, for two reasons.  First, mpg easily tells you what you most want to know, e.g. if I have 5 gallons in the tank, and I see a sign saying "next gas 150 miles", it's pretty easy to figure that if I get 30 mpg or less, I'd better not try to make it.

Second is that gallons per 100 miles is just plain silly.  Gallons per mile, maybe, possibly even gallons per 1000 miles, but why on earth the odd factor of 100?

[Portland Man]

I disagree, for two reasons.  First, mpg easily tells you what you most want to know, e.g. if I have 5 gallons in the tank, and I see a signs saying "next gas 150 miles", it's pretty easy to figure that if I get 30 mpg or less, I'd better not try to make it.

Second is that gallons per 100 miles is just plain silly.  Gallons per mile, maybe, possibly even gallons per 1000 miles, but why on earth the odd factor of 100?

True MPG is more easily converted to miles until empty, but it sucks for comparing two different cars.
Does it save more money to go from 15 to 25 mpg or to go from 25 mpg to 50 mpg?  25/15 is less than double, and 50/25 is doubling your miles per gallon, the latter must be better, right?  By converting to gallons per 100 miles you'll see going from 15 to 25 mpg you save 2.67 gallons/100 miles, and going from 25 to 50 mpg you only save 2 gallons/100 miles.

Why per 100 miles?  Why not?  Per 1,000 would be fine too, the main thing is that miles per 100 yields a number that is reasonably easy to deal with.

25 mpg = 0.04 gal/mile = 4 gal/100 miles
35 mpg = .028 gal/mile = 2.857 gal/100 miles

4 and 2.86 are just easier for the mine to work with.  40 and 28.57 gal/1,000 miles are reasonably easy as well.

[Jamesqf]

True MPG is more easily converted to miles until empty, but it sucks for comparing two different cars.

But which do you do more often?  You probably make decisions on whether to fill up now or wait every week or so, but need to compare cars once every few years.

Does it save more money to go from 15 to 25 mpg or to go from 25 mpg to 50 mpg?

The correct answer, as I always point out when someone asks me this trick question, is to go from 15 to 50 mpg.  And that is something just as easily seen from mpg as from g/100m :-)

[AmbystomaOpacum]

How often do you actually know how much fuel you have? My fuel meter is way too erratic (read: compensated to provide drivers with the kind of feedback auto makers think they want) to realistically estimate how much fuel I have left.

I fill up when it's convenient to do so and I have less than half a tank. I never think about how many miles I might have left to drive.

And either way, the OP said this thread was about "fuel economy", and for that, MPG is not nearly as useful as expected gasoline cost per x miles.

[tooqk4u22]

How often do you actually know how much fuel you have? My fuel meter is way too erratic (read: compensated to provide drivers with the kind of feedback auto makers think they want) to realistically estimate how much fuel I have left.

I fill up when it's convenient to do so and I have less than half a tank. I never think about how many miles I might have left to drive.

And either way, the OP said this thread was about "fuel economy", and for that, MPG is not nearly as useful as expected gasoline cost per x miles.

Your car won't do this for you...all you have to do is fill up your tank, record mileage, drive....next time fill up tank (this is how many gallons you actually used), subtract current mileage from last mileage (this is how far you actually drove) then calculate.

[Russ]

Russell touched on this already, but I think the difference is that with MPG you are "getting" x number of miles for every gallon, whereas when using gal/100mi. you are "giving" x number of gallons to go 100 miles. Which one do you think the auto industry would rather have you think? My bet's on what you "get" by using a car, or MPG.

[grantmeaname]

doubling MPG does not halve fuel use

Yes it does. A 25mpg car can go 100 miles on 4 gallons of gas. A 50 mpg car can go 100 miles on 2 gallons of gas. In the opposite direction, a 12.5mpg car can go 100 miles on 8 gallons of gas.

[AmbystomaOpacum]

Your car won't do this for you...all you have to do is fill up your tank, record mileage, drive....next time fill up tank (this is how many gallons you actually used), subtract current mileage from last mileage (this is how far you actually drove) then calculate.

Sure, but... gas stations are everywhere, so I still don't see why I'd care whether I have 100 or 150 miles left. It's less than half, I fill up. Then I have plenty.

[velocistar237]

As long as you're switching, you might as well go metric with L/100km.

Sure, but... gas stations are everywhere, so I still don't see why I'd care whether I have 100 or 150 miles left. It's less than half, I fill up. Then I have plenty.

In a 3,000 pound car, each 30 pounds you slice from the weight cuts your weight by 1% and increases your fuel efficiency by about the same amount....
A full tank of gas at 8 pounds per gallon usually weighs over a hundred pounds too. Because of this, and because a full tank of gas lasts several months in my van (raising concerns about spoilage), I usually buy only 1/4 to 1/2  a tank.

[Jamesqf]

Sure, but... gas stations are everywhere, so I still don't see why I'd care whether I have 100 or 150 miles left.

Maybe that's true in the southeast US.  In the west, it's not that uncommon to see signs like this: http://www.drfphoto.com/photo/23722/  Longest stretch I know of on paved roads is about 160 miles (between Tonopah and Ely, in Nevada), and getting off pavement can increase that quite a bit.