MPGe for the Dashboard

It was interesting and fun to watch the efficiency tests during the Knockout Qualifying Stage of the Progressive Insurance Automotive X PRIZE:

   

Watching this event reminded me that, as more alternative-fuel and high-efficiency vehicles come to market,  consumers will want dashboard instrumentation that helps them to maximize efficiency.   What information would best serve that purpose?   

Years ago, efficiency-conscious drivers had to write down odometer and pump readings when buying gasoline, and then do the math.   Today, many vehicles include real-time dashboard displays of instantaneous and average miles-per-gallon (MPG), something that efficiency-conscious drivers appreciate and make use of.  

Unfortunately, while it’s simple and well-understood, MPG is obsolete for two main reasons: the growing popularity of alternative fuels, and the emergence of vehicles powered by multiple fuels (most importantly, plug-in hybrids that use electricity plus gasoline or another liquid fuel). 

Comparing the MPG of gasoline with the MPG of ethanol (to use one example) is like the proverbial comparison of apples to oranges; besides, what’s a gallon of electricity?   For a Plug-in-Hybrid-Electric-Vehicle (PHEV), MPG is even more misleading (at best, it tells just part of the story – e.g., accounting for just the liquid fuel but not the electricity).  

If you’ve been following the Progressive Insurance Automotive X PRIZE, you know that we think that the best replacement for MPG is MPGe – miles per gallon energy equivalent), defined as

MPGe =  (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)]

Like MPG, MPGe is a pump (or plug!) to wheels measure – it only depends on the inherent efficiency of your vehicle and how you drive it.   Some have argued that “MPGe” should be defined as a well-to-wheels measure that also accounts for upstream energy conversion losses during fuel production and transportation.  While such well-to-wheel measures indeed have their place, the dashboard isn’t one of them.  

Consumers understand pump-to-wheels measures – nobody does a well-to-wheels calculation when they refuel. Furthermore, upstream fuel efficiencies are ambiguous as they involve controversial and changing-in-time assumptions and predictions on which proponents of different fuels rarely agree.  Note also that that well-to-pump efficiencies are in many cases addressed indirectly by the Progressive Insurance Automotive X PRIZE requirement on total well-to-wheels CO2 emissions.

MPGe is a simple, well-defined measure of overall vehicle efficiency.  In effect, it replaces the question

“How far can I drive on a fixed amount of gasoline?”

with the question

“How far can I drive on a fixed amount of energy?”.  

MPGe is easy to explain, depends only on your vehicle and how you drive it, accounts in a neutral manner for any combination of fuels, and reduces to the familiar MPG in the case of gasoline fuel only.  To further explore MPGe, check out the spreadsheet available here.

MPGe can be calculated easily from on-board measurements of fuel(s) flow and distance traveled.   Here’s hoping that you’ll see it on your dashboard soon. 

Comments

[Jim Bullis, Miastrada Company]

It looks like mind control has set in at xprize.