Quote:
Originally Posted by Atheria
I was totally excited when after my first 117 miles, I got 48.5 mpg!
...
I was expecting to get 50 mpg on this fill-up, but nooooo....I averaged only 38.2 mpg!
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***warning, long boring math ahead***
***Short version for the tl;dr crowd (apologies to people who have already seen this in my last 239023094 posts, it's a pet peeve on mine) *** changes in MPG can be misleading. You dropping 10 MPG isn't the same increase of fuel consumption as someone going from 100 MPG to 90 MPG (smaller increase of fuel consumption as you) nor 20 MPG to 10 MPG (larger increase of fuel consumption as you)
***long version starts here***
Just to put things into perspective, because changes in MPG can be misleading, these are the MPG figures converted into gallons used per 100 miles (G/100M):
48.5 MPG = 2.06186 G/100M
38.2 MPG = 2.61780 G/100M
That change of 10.3 MPG was an increase of approx 0.556 gallons per 100 miles. If you traveled the same 117 miles at the same MPG, you consumed 3.06283 gallons on the 2nd tank and 2.41238 gallons on the 1st one, for only a difference of 0.65045 gallons (about $2.458 more in gas at $3.779 a gallon)
If we apply that 0.556 G/100M difference in fuel consumption to a different car:
22.50225(repeating) MPG = 4.444 G/100M
20.0 MPG = 5.0 G/100M
That is a difference of 0.556 as well. So you're 10 MPG drop is the same as someone else's 2.50225(repeating) drop in MPG. Same amount of increase in fuel consumption, just a difference in expression of distance traveled per set volume instead of volume of fuel consumed over a set distance.
At the other extreme, it would be like someone who got 100 MPG (1 G/100M) dropping to 64.26735 MPG (1.556 G/100M) because their fuel consumption also increased 0.556 gallons per 100 miles.
Moral of the story: Do not get so hung up on MPG changes because they are not comparable to each other. Your 10 MPG drop is someone else's 35.7326 MPG drop and a different person's 2.50225 MPG drop. All of them consumed the exact same amount of extra fuel per set distance.