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				Compare your car
Model	Toyota Prius Plugin	Toyota Prius	US fleet average	Choose a state Alabama Alaska Arizona Arkansas California Colorado Connecticut Delaware Florida Georgia Hawaii Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvania Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virginia Washington West Virginia Wisconsin Wyoming U.S. Average Off-Grid (wind, solar)
MPG	57.6	42.1	19.81
Wh/mile	131.5	—	—
CO2e lbs/mile2	0.4745	0.5604	1.1923
CO2e emissions saved	60%5	53%4	—
Gallons of gasoline saved per year6	398	321	—
Barrels of oil saved7	66%	53%	—
Percent of US fleet to halve CO2e emissions8	83%	94%	—
Cost savings in dollars per year9	$1493	$1333	—

Understanding the Calculations

1. US Fleet Average MPG

The average combined MPG for all US cars and light trucks on the road today is 19.8 MPG. (Source: 2005 Highway Statistics from the U.S. Department of Transportation, Federal Highway Division).

2. What is CO₂e?

CO₂e is a measure that expresses the amount of greenhouse gases produced in terms of the amount of carbon dioxide (CO₂) that would have the same global warming potential. This allows a single value to encompass the effects of many gases, such as methane, perflouorocarbons, and nitrous oxide, in addition to just carbon dioxide.

3. CO₂e Emissions for US Fleet Average

CO₂e benefits shown in the calculator are derived relative to the US fleet average. The US fleet average is 14,303 pounds/year CO₂e for 12,000 miles or 1.192 pound CO₂e per mile. The calculation is:

12,000 miles ÷ 19.8 mpg = 606.1 gallons
606.1 gallons x 23.6 pounds CO₂e/gallon = 14,303 pounds CO₂e/year
23.6 pounds CO₂e/gallon ÷ 19.8 MPG = 1.192 pounds CO₂e/mile

Each gallon of gasoline burned includes 19 pounds CO₂e/gallon for tank-to-wheels fuel use and 4.6 pounds CO₂e/gallon for upstream well-to-tank emissions. The source of the gasoline CO₂e emission factor is chemical mass balance for converting gasoline to CO₂ and Technology and Cost Assessment for Proposed Regulations to Reduce Vehicle Climate Change Emissions Pursuant to Assembly Bill 1493, California Air Resources Board, 4/1/2004.

4. CO₂e Emissions for Conventional Prius

CO₂e emissions for the conventional Prius in our fleet (without the plug-in conversion) is 6,924 pounds/year for 12,000 miles or 0.577 pounds CO₂e/mile. This number is based on the 49.0 MPG observed (as of 7/18/2008) from real use data from our data acquisition system as described in the methodology section. The calculation is as shown above for the US fleet average, except that it uses the actual mileage driven and the observed MPG.

The percentage of reduced CO₂e emissions is calculated as for the US fleet average, above, except that it uses the 6,924 pounds CO₂e/year computed for the Prius.

5. CO₂e Emissions for Prius Plug-ins

CO₂e emissions for the plug-in hybrids in our fleet (as of 7/18/2008) are 4,860 pounds CO₂e/year for 12,000 miles or 0.405 pounds CO₂e/mile. Total CO₂e emissions are the sum of CO₂e emissions from gasoline use and electricity use. Gasoline use CO₂e emissions are calculated using the methodology previously described above for the conventional Prius.

Electricity emissions are calculated by the following equation:

Total kWh/mile x state-level emission factor (pounds CO₂e/kWh) = pounds CO₂e per mile electricity use.

The percentage of reduced CO₂e emissions is calculated as for the US fleet average, above, except that it uses the 4,860 pounds CO₂e/year (as of 7/18/2008) computed for the plug-in Prius.

6. Gallons of Gasoline Saved Per Year

We compare gasoline used for the comparison cars (conventional Prius, plug-in Prius, and your car) to the average US fleet vehicle for a 12,000 mile annual driving cycle. The calculation is:

average gallons used = 12,000 / average MPG
comparison gallons used = 12,000 / comparison MPG
gallons saved = comparison gallons used - average gallons used

7. Barrels of Oil Saved

One barrel of oil yields approximately 20 gallons of gasoline. To calculate barrels of oil saved, we compare gasoline used for the comparison cars (conventional Prius, plug-in Prius, and your car) to the average US fleet vehicle for a 12,000 mile annual driving cycle. The calculation is:

average barrels used = 12,000 / (20 * average MPG)
comparison barrels used = 12,000 / (20 * comparison MPG)
percentage saved = 100 * (1 - comparison barrels used / average barrels used)

8. Percent of US Fleet to Halve CO₂e emissions

The average car in the US gets 19.8 MPG and produces 1.192 pounds of CO₂e per mile. Given the reduced emissions of hybrid and plug-in hybrid vehicles, we calculate the percentage of such cars we'd need to reduce overall CO₂e emissions by half.

As calculated above (as of 7/18/2008), a Prius plug-in gets 69.2 MPG and uses 129.7 Wh/mile. Overall it produces 0.405 pounds CO₂e per mile. To halve average emissions, we want:

P x 0.405 + (1 - P) x 1.192 = 1.192 ÷ 2

where P is the percentage of Prius plug-ins. In this case, P is 0.76, or 76%.

The calculation is similar for a conventional Prius, except that it generates 0.481 pounds CO₂e per mile (as of 7/18/2008).

9. Cost Savings

This calculation assumes that gasoline costs $4.15 per gallon, and that electricity costs $0.10 per kWh. Assuming a 12,000 mile yearly driving cycle, the yearly fuel cost to operate a gasoline vehicle or conventional hybrid is:

cost = 4.15 x 12,000 ÷ MPG

So, for example, the average US car that gets 19.8 MPG would cost 4.15 x 12,000 ÷ 19.8 = $2525.15 per year for fuel.

For a plug-in hybrid, we add the cost of electricity:

cost = 4.15 x 12,000 ÷ MPG + 0.10 x 12,000 x kWh/mile

A Prius plug-in that gets 69.2 MPG and uses 0.1297 kWh/mile (129.7 Wh/mile) would cost 4.15 x 12000 ÷ 69.2 + 0.10 x 12000 x 0.1297 = $875.29 per year, for a savings of $1649.86 per year when compared to the average car.