What's the average of 5 MPG and 10 MPG?
The math isn't quite obvious to me, but if you go out camping and get 5 MPG on the way to the campground and turn around and get 10 MGP on the way back, your average MPG won't be 7.5 as you would expe...
Forum Discussion
holstein13 wrote:
The math isn't quite obvious to me, but if you go out camping and get 5 MPG on the way to the campground and turn around and get 10 MGP on the way back, your average MPG won't be 7.5 as you would expect. It's actually 6.67.
In other words, you used two thirds of your fuel on the way up and one third on the way back.
Which gets me to thinking, why don't we simply use fuel consumption (i.e. gallons per hundred miles) instead of miles per gallon? It would be much easier to compare mileage if we did so.
7.5 and 6.67 are pretty far apart, but so is 5 mpg and 10 mpg.
Let's say you got 7 mpg on the way there, and 8 mpg on the way back....
(7+8)/2 = 7.5 mpg. Taking the average of averages gives the "incorrect answer".
Doing the match correctly...
(50/7) + (50/8) = 13.392985 gallons
100/ 13.392985 = 7.47 mpg.
The difference between 7.5 and 7.47 is pretty negligible. Getting 7 mpg one way, and 8 mpg the other way is perhaps more realistic that 5 and 10, respectively. That is, unless you went 50 miles up a steep hill and then came back down, but that's also not a great way to gauge fuel economy. With that in mind, the method of calculation becomes less important because the two methods yield very similar results in real world conditions.
