MPG is stupid

[kevm14]

Dan Edmunds is preaching for standardization around the gallons per 100 miles format. He makes a good argument and it revolves around the psychology of math.

http://www.edmunds.com/car-reviews/feat ... tupid.html

I particularly like the first example. We already knew to go on percentages (i.e. going from 20 to 25mpg is the same percentage improvement as 30 to 37.5mpg). But he makes a good point that it's more than percentages. A vehicle getting 20mpg is consuming 5 gallons per 100 miles. If another vehicle gets 25mpg, it consumes 4 gallons per 100 miles. So the first vehicle consumes 1 gallon more per 100 miles. On a notional 1000 mile trip, the first vehicle will consume 10 additional gallons. If gas costs $3.50/gal, that's an increase of $35.

Now take the second pair of vehicles. The vehicle getting 30mpg is consuming 3.33 gallons per 100 miles. The second vehicle in this pair gets 37.5mpg which is 2.67 gallons per 100 miles. Again, the first vehicle consumes 0.67 additional gallons per 100 miles. On that notional 1000 mile trip, the first vehicle will consume 6.7 additional gallons, which is an increase of $23.45.

But wait! Using our simple percentages method (which we already thought was compensating for the non-linearity of MPG), the 25% improvement in MPG for each car pair results in diminishing actual fuel savings, depending on what MPG you apply that 25% to.

Put another way, 25% over 20mpg is 25mpg. 5 mpg. So 25% over 30mpg is 37.5. 7.5mpg. Apples to apples? Nope. Your 20 to 25mpg improvement was worth $35 in fuel savings. To get the SAME savings starting from 30mpg, you would need to NOT simply improve by the same 25%. To save $35, we need to save 10 gallons. Work the math backwards: On the 1000 mile trip, our 30mpg car uses 3.33 gallons per 100 miles. So that's 33.3 gallons. Subtract 10 = 23.3 gallons used. If we used 23.3 gallons over 1000 miles, that's 2.33 gallons per 100 miles of fuel economy that we need to achieve $35 savings. 2.33 gallons per 100 miles translates to 43mpg.

So our 5mpg improvement going from 20 to 25mpg is worth the exact same in fuel savings as going from 30 to 43mpg.

That's kind of interesting, don't you think? It likely means chasing mpg gains if you're already at, say, 30, is not nearly as impactful as you think, even if you try to use percentages instead of linear MPG.

Discuss.