The impact of A/C on MPG

Huh?

By whose definition is the reciprocal operation a linear function?

 

You are correct. It is less an issue of linearity... but the US system is flawed in that it places the cost (as in gallons of gas) in the denominator. This is a vastly inferior method of determining economy, and out of sync with the majority of the rest of the world.

There was a great conversation on "Car Chat" that I will do my best to re-create.

• You have two cars... one driven by you, the other by your partner
• Both are driven the exact same miles every day. In opposite directions
• This usage will continue irrespective of the choice you will be offered
• One car gets 10 MPG, the other get 100MPG
• You can upgrade one (and only one) of the two cars
  • Option 1: Upgrade the 10 MPG car to 11 MPG
  • Option 2: Upgrade the 100 MPG car to 200 MPG
• Which do you choose

Most people in the US would intuitively choose Option 2 (unless they were moderately good at math)... but most people in the rest of the world would choose the much better option 1, which would save them about twice as much money.

This is an artifact of the inferior MPG method we use... were the $ is in the denominator. The rest of the world primarily uses liters/100KM... putting the dollars in the numerator.

So you are correct... the issue is less one of linearity... but rather, it is the nature of the inverse function instead. In any case, I think the person you quoted was highlighting how the US fuel efficiency reporting is seriously flawed vs the rest of the world.

/Jim

 

I would choose option 3, trade the 10 mpg car for a 200 mpg one
I'd love to own a car that does 200 mpg.

 

Help me out here.

Miles per gallon. Liters per 100 kilometers.

I see a distance per a standard unit of fuel and amount of fuel per standard units of distance.

I don't see currency/dollars in either format. Merely two formats flipped. Neither expresses the cost per unit of distance.

 

Cost is a pretty direct function of the quantity of fuel purchased.

In the Car-Talk interview example that I used... it is clear that the rest of the world would clearly see a "quantity of gasoline used" (which I conveniently state is a $$$ statement)... vs the MUCH less useful USA method of MPG.

We (USA) made a poor choice. We may not notice it because the current measurement method is ingrained. However, that doesn't mitigate the fact that we made a poor choice.

/Jim

 

Miles per gallon is equivalent to saying how many miles you get for a $, e.g. if it's $4 a gallon and you get 50 mpg, then you get 12.5 miles per $.
and from Litres/100km you can work out how many $ it costs per mile.
If I worked it out right, at $4 a gallon (US), 5.3litres/100km would cost you $0.08 per mile (equivalent to 50 mpg).