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MPG Illusion

 


MPG Illusion

Putting efficiency aside for the moment, I'd like to present some simple arithmetic to show that really high gas mileage isn't all it's cracked up to be. I'll show how to find out what the hybrid payoff really is by calculating gallons per mile (GPM or GPKM).

Going from a 30mpg car to a 50mpg car will save $5,320 on $4.00/gallon gasoline every 100,000 miles.

Going from a 10mpg car to a 30mpg car will save $26,800 on $4.00/gallon gasoline every 100,000 miles.

Wait! What? There was only a 20mpg improvement between each case! What's going on?!

Comparing cars by MPG does not really show how much fuel a car consumes. The relationship between MPG and fuel consumption is not linear. More fuel can be saved by changing from a 10mpg car to a 20mpg car than from going from a 20mpg to a 100mpg car.

What gives? That chart can't possibly be right!

MPG is misleading because it is a rate of consumption. MPG is a good metric for monitoring driving habits. However, MPG is not a good metric for choosing a car.

The chart below may shed some light on the situation. The column on the left shows what we all know to be an accurate way to measure fuel consumption. The column on the right shows how much fuel is consumed at that rate for a given distance of 1000 miles.

MPG
Gallons per 1000 miles
10
100
15
67
20
50
25
40
30
33
35
29
40
25
45
22
50
20
55
18
60
17
65
15
70
14
75
13
80
13
85
12
90
11
95
11
100
10

GPM (gallons per mile) shows how much fuel a car uses over a fixed distance. I arbitrarily chose a fixed distance of 1000 miles because the result was usually a simple two-digit number. Calculating GPKM is easy. GPKM=1000miles/?mpg. Just fill in the "?" with the average mpg value of the car you're looking into. For example, a 30mpg car: 1000/30mpg = 33 gallons for every 1000 miles.

To clear up any confusion, I will use GPKM to mean "gallons for every 1000 miles." GPM does literally mean "gallons per mile," although I may imply GPKM.

The graph below is a visual representation of the chart above. This is a very clear picture that shows the non-linear relationship between MPG and GPKM.

But I still want a 100mpg car!

The trouble with achieving such high gas mileage is cost. A lot of exotic technology is required to build a vehicle that can get that kind of gas mileage. The benefit probably won't pay the cost. And the cost for that sort of efficiency can be had for about $100,000 in the Tesla Roadster or similar electric cars. As battery technology develops, the cost of exotic batteries that power these cars may fall from the $10,000 to $20,000 price range. $20,000 is a lot to pay for what is basically a one-gallon gas tank. A $2 or $5 gallon of gas is dirt-cheap by comparison.

So how much fuel can be saved from trading in a thirsty car for a gasoline camel?

A hybrid system will typically reduce fuel consumption at the expense of performance such as acceleration, braking, and handling. Keep in mind that hybrids still do get ALL of their energy from gasoline. There isn't much fuel left to save after 40mpg.

How does a Camry Hybrid compare to its four-cylinder sibling, a Camry with a 2.4L gas engine?

A 2008 Camry Hybrid averages 33.7mpg and costs $30,500.

A 2008 Camry with a 2.4L gas engine averages 25.7 mpg and costs $23,000

A quick look may convince someone they would save 31%
(33.7-25.7mpg)/25.7mpg more on gas money by going with the hybrid. But they would really be saving about 24% (39-30gpkm)/39gpkm.

 
2008 Camry Hybrid
2008 Camry 2.4L

Cost

$30,500
$23,000
Avg. MPG
33.7 mpg
25.7 mpg
GPKM
30 GPKM
39 GPKM
 
$2/gallon
$59
$78
100,000 mi.
$5,935
$7,782
$4/gallon
$119
$156
100,000 mi.
$11,869
$15,564

The Camry Hybrid costs about $7500 more than the 2.4L gas engine Camry. Over 100,000 miles of ownership of $4.00 per gallon gasoline, the Hybrid saves about $3,700 in gas money. The savings covers half the difference in the price of the car only for $4.00 per gallon gasoline. At $2.00 per gallon of gasoline, the hybrid saves about $1800 in gas money. Either way, the Hybrid would need to drive 200,000 to 400,000 miles to break even with the cost difference.

This assumes that the $8,000 battery doesn't need to be replaced during that time.

As gas mileage increases, the potential return on investment declines quickly. Adding up all the savings from 20mpg to 100mpg saves 40 gallons per 1000 miles.

The biggest difference this country can make to reduce the gasoline consumption in the transportation sector is to trade in or modify 10mpg cars to achieve 20+mpg.

Still not completely convinced? Read the MPH Illusion. It's the same math applied over a more familiar subject.

 

Let's Review

MPG numbers are misleading.

Hybrid systems do not pay for themselves in gas savings.

If choosing a car for gas mileage, just aim for the 30mpg range.

If your car gets 20mpg or more, that's great!

Don't pay over $1000 to get more than 25mpg. The cost will not outweigh the benefit.

If you want to save the planet, your hybrid money would be better spent on conservation efforts.

If you want to take that 20mpg car and make it a 30mpg car, click here to read about the various ways that can be done.

Read about:

What is an economical car?

The MPH Illusion

Toyota Prius

Tesla Roadster

Acceleration Test

Hydrogen Fuel Cells and the Honda FCX Clarity

Find out how to improve gas mileage in your car

 

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