Understanding fuel economy: the problems with MPG
Lately, there's been some discussion of how we measure fuel economy. The argument is that the current measuring yard of miles per gallon (MPG) can be deceiving. Consumers assume that ever-greater MPG ratings yield ever greater savings. The truth is a little more complicated.
In this article, we'll look at fifteen cars ranging in fuel economy from 15 to 52 miles per gallon. We'll compare them using different benchmarks to gain a better understanding of how costs change with MPG.
Miles per gallon (MPG)
First, we'll look at the standard miles per gallon measurement.
|2017 Toyota Tundra 4WD||15||-||-|
|2017 Lincoln Navigator L 4WD||16||1||6%|
|2017 Land Rover Range Rover Sport||20||4||20%|
|2017 Audi Q7||22||2||9%|
|2017 Volvo XC90 AWD||23||1||4%|
|2017 Mazda CX-5 AWD||26||3||12%|
|2017 Mercedes-Benz CLA250 4MATIC||37||1||4%|
|2017 Volkswagen Golf||29||2||7%|
|2017 Subaru Impreza 5-door||31||2||6%|
|2017 Audi A3 e-tron||34||3||9%|
|2017 Honda Civic||36||2||6%|
|2017 Lexus ES 300h||40||4||10%|
|2017 Chevrolet Volt||42||2||5%|
|2017 Honda Accord Hybrid||48||6||13%|
|2017 Toyota Prius||52||4||8%|
There's a disconnect between the increase in miles per gallon rating and the percent by which the actual fuel economy changes. Looking more closely at the three instances where MPG rating goes up by 4, we see that this represents a 20% increase when MPG rises from 16 to 20, but only a 4% increase when MPG rises from 48 to 52. We'll explore more about what this means below.
Gallons per 100 miles
Next, we'll look at what many are proposing as an alternative to miles per gallon—gallons per 100 miles.
|2017 Toyota Tundra 4WD||15||6.67||-|
|2017 Lincoln Navigator L 4WD||16||6.25||0.42|
|2017 Land Rover Range Rover Sport||20||5.00||1.25|
|2017 Audi Q7||22||4.55||0.45|
|2017 Volvo XC90 AWD||23||4.35||0.20|
|2017 Mazda CX-5 AWD||26||3.85||0.50|
|2017 Mercedes-Benz CLA250 4MATIC||37||3.70||0.14|
|2017 Volkswagen Golf||29||3.45||0.26|
|2017 Subaru Impreza 5-door||31||3.23||0.22|
|2017 Audi A3 e-tron||34||2.94||0.28|
|2017 Honda Civic||36||2.78||0.16|
|2017 Lexus ES 300h||40||2.50||0.28|
|2017 Chevrolet Volt||42||2.38||0.12|
|2017 Honda Accord Hybrid||48||2.08||0.30|
|2017 Toyota Prius||52||1.92||0.16|
Examining the same three instances where MPG rating goes up by 4, the gallons per mile number decreases by 1.25 gallons when MPG rises from 16 to 20, but only by 0.16 gallons when MPG rises from 48 to 52. By flipping the numbers, it becomes a little more intuitive that there are diminishing returns on increasing MPG ratings
Cost per year
Finally, we'll look at what it might actually cost to drive each vehicle for a full year. We've assumed 12,000 miles are driven per year and that regular gasoline is purchased at the California average price of $2.975 (at the time of writing).
|2017 Toyota Tundra 4WD||15||$2,380||-|
|2017 Lincoln Navigator L 4WD||16||$2,231||$149|
|2017 Land Rover Range Rover Sport||20||$1,785||$446|
|2017 Audi Q7||22||$1,623||$162|
|2017 Volvo XC90 AWD||23||$1,552||$71|
|2017 Mazda CX-5 AWD||26||$1,373||$179|
|2017 Mercedes-Benz CLA250 4MATIC||37||$1,322||$51|
|2017 Volkswagen Golf||29||$1,231||$91|
|2017 Subaru Impreza 5-door||31||$1,152||$79|
|2017 Audi A3 e-tron||34||$1,050||$102|
|2017 Honda Civic||36||$992||$58|
|2017 Lexus ES 300h||40||$893||$99|
|2017 Chevrolet Volt||42||$850||$43|
|2017 Honda Accord Hybrid||48||$744||$106|
|2017 Toyota Prius||52||$687||$57|
Once more, we'll look at the cases where MPG rating goes up by 4. A change from 16 to 20 mpg yields $446 in annual savings, while a shift from 48 to 52 mpg saves only $57 per year.
MPG vs. Gallons per 100 miles
Switching from miles per gallon to gallons per 100 miles could potentially help on the lower side of the spectrum. The driver who is considering a Lincoln Navigator and a Range Rover Sport might be more willing to switch to the more fuel efficient vehicle when presented with a difference of 1.25 gallons per 100 miles (or $446 per year) rather than 4 miles per gallon. However, it could also make the consumer considering the Prius and the Accord Hybrid to opt for the lower fuel economy vehicle.
In general, we're pushing towards higher fuel economies and lower gas consumption. As this technology continues to improve we should see the average fuel economy rise no matter which standard we choose to measure it by.