Miles per Gallon versus Miles Per Dollar


A new unit of measurement to gauge a car's fuel economy

Miles per gallon (MPG) is the common and widely accepted unit of measurement for assessing a car's fuel economy. So prevalent, MPG estimates are included on the window sticker of virtually every new and used car available on the market today. Simply put, MPG is the measurement of the number of miles a vehicle can travel on one gallon of gas. However, given the current economic climate and our greater awareness of fuel efficiency, there may be a better unit of measurement — one that actually puts a price on the miles we drive.  Known as miles per dollar (MPD), this emerging mathematical equation offers a different take, and perhaps a better one, on measuring fuel economy.

Out with MPG, in with MPD?

For decades, we've become accustomed to thinking in terms of MPG.  Legislation and governmental controls for cars have used it as a primary marker. We've labeled cars with higher MPG ratings as 'gas sippers' and cars with lower MPG ratings as 'gas guzzlers'. However, miles per dollar could help us think of driving in a whole new light.  Instead of making fuel consumption data the primary benchmark, MPD holds people accountable for the driving choices they make.  Philosophically speaking, MPD measures drivers while MPG measures cars. The hope is that, when using MPD, we'll start making better driving decisions, such as driving less often, thinking twice about making unnecessary trips, choosing driving destinations closer to home and ultimately, buying more fuel efficient cars.

For example, your car might get 20 MPG, but when you think in terms of MPD, driving seven miles would cost you approximately one dollar. What if you drive a gas guzzler?  What if you have a lead foot?  Depending on how far you drive, imagine what adding an extra $5, $10 or $15 in fuel costs would mean when driving to the grocery store, to a restaurant, to church, to work or to a vacation destination? The point is, when we calculate MPD, we calculate the dollars spent versus calculating the fuel consumed.

The math of MPD

From a rudimentary perspective, the math for calculating MPD is to take your car's current EPA estimates and divide by the current price per gallon of gasoline. Some of us may remember when gas was about $1 per gallon (ah, the good old days). For a car with 18 MPG, this meant we could travel 18 miles for $1. Nowadays, gas costs have nearly tripled, which means that a car with 18 MPG would require $1 for about every 6 miles driven.  With these numbers, that 300 mile driving vacation you've planned will end up costing you about $50 in fuel costs alone. 

Is MPD practical?

It's uncertain whether or not MPD will ever catch on as an official unit of fuel economy measurement.  One major disadvantage of the formula is that fuel prices are a constantly moving target. Therefore, your car's MPD changes every time the price of gasoline does. What's more, MPD fails to calculate the true cost of driving your car by omitting expenses for insurance, repairs and registration costs.  But then again, so does MPG.

Informally, MPD helps us gain a better perspective of the cost of driving our cars.  What's more, it puts a hard dollar amount on driving which, in turn, enables us to factor these costs into our monthly driving budgets. Finally, by knowing this hard dollar amount, MPD enables us to modify our driving behaviors — to make practical changes to the way we drive, to where we drive and to how much we drive — to help us save money at the pump.  After all, isn't that what a solid unit of measurement is supposed to do?

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