The Chevy Volt was used to illustrate this method.

In May 2012, the US finalized new standards to increase the corporate average fuel economy, CAFE, of cars and light trucks from the current 27.5 mpg to 35.5 mpg by 2016 and 54.5 mpg by 2025.

Assumptions:

The US power system CO2 emission intensity is 1.341 lb CO2/kWh.
Gasoline CO2 emission is 19.4 lb/gal.
Present CAFE = 27.5 mpg; 2025 CAFE = 54.5 mpg
Chevy Volt (AC/DC conversion + charging) loss at 120 Volt is 16.1%, and at 240 Volt 8.9%
Chevy Volt “mileage” in EV mode is 0.30 kWh (DC)/mile (at the battery), or 0.3721 kWh (AC)/mile (produced by the power plant).
Chevy Volt mileage in gasoline mode is 37 mpg; EPA combined.
http://www.fueleconomy.gov/feg/noframes/31618.shtml

...
Calculations:

Present CAFE CO2 emissions = 19.4 lb CO2/gal x 1gal/27.5 m x 454 g/lb x 1 m/1.609 km = 199.0 g/km

2025 CAFE CO2 emissiions = 27.5/54.5 x 199.0 g/km = 100.5 g/km

If 100% of the Chevy Volts were charging at 240 V, it would require 0.30 x 1.089 (AC to DC conversion + charging @ 240 V) x 1.05 (T&D) x 1.05 (Self-use) = 0.3602 kWh (AC)/mile to be produced by the US power system.

If 50% were charging at 120 V and 50% at 240 V, it would require 0.5 x 0.3602 + 0.5 x (1.161/1.089) x 0.3602 = 0.3721 kWh (AC)/mile to be produced by the US power system.
http://theenergycollective.com/willem-post/171561/co2-emissions-and-chevy-volt-vs-honda-civic-ex-l

Chevy Volt CO2 emissions in EV mode = 0.3721 kWh/mile x 1.341 lb CO2/kWh x 454 gr/lb x 0.6214 m/km  = 140.8 gr/km.

Chevy Volt CO2 emissions in gas mode = 19.4 lb CO2/gal x 454 gr/lb x 1 gal/37 mile x 0.6214 m/km = 147.9 gr/km

Below is a comparison of 4 cases of travel.

Travel                  Mileage        Energy                   Emissions
miles/yr                  mpg                              lb CO2/yr      g CO2/km

Present CAFE                  12,000                  27.5          436 gal           8,465          199.0
2025 CAFE                     12,000                  54.5          220 gal           4,272          100.5
Chevy Volt, EV mode       12,000                                4,465 kWh         5,748          140.8
Chevy Volt, Gas mode      12,000                  37.0          324 gal           6,292          147.9

The above table indicates, the Chevy Volt is slightly more efficient in EV mode than in gas mode regarding grams of CO2 emissions/km. The CO2 either comes out of a tail pipe, or out of a power plant chimney.

METHOD No. 2: BASED ON EPA's APPROACH

The “mpg equivalent” metric, introduced in November 2010 by the EPA, is based on 33.7 kWh per gallon of gasoline. The energy consumption of a vehicle is determined by the EPA's five standard drive cycle tests simulating varying driving conditions. All new cars and light-duty trucks sold in the U.S. are required to have a label showing the EPA's estimate of fuel economy of the vehicle.
http://en.wikipedia.org/wiki/Miles_per_gallon_gasoline_equivalent

Chevy Volt in EV mode uses 35 kWh/100 miles, per EPA.
Chevy Volt EPA mpg-e = 100 m/35 kWh x 33.7 kWh/gal = 96.3 mpg-e. (the below website states 98 mpg-e.
Chevy Volt in gas mode uses 37 mpg, per EPA
http://en.wikipedia.org/wiki/Plug-in_hybrid

The EPA method has no basis in physical reality regarding CO2 emissions/mile, because it equates the thermal energy in a gallon of gas with the electrical energy produced by a power plant; it would take the thermal energy of about 3 gallons of gasoline to produce 33.7 kWh.

The EPA method is highly deceptive, because it causes people to think millions of plug-in hybrids would be a significant factor in reducing CO2 emissions, whereas, in fact, plug-in hybrids would hardly make a difference, as shown under Method No.1.

But, the EPA method does serve a political purpose, as it allows manufacturers to use the high mpg-e values for calculating their CAFE values, which would make it much easier to comply with the 2025 CAFE standard; a loophole designed large enough for an 18-wheeler.

Note: Earlier CAFE standards were rife with loopholes which resulted in a difference between the EPA test mpg and real-world mpg of up to 20%. Washington’s “constituent service providers” should be less lenient in the future.

http://theenergycollective.com/jemillerep/104841/can-new-cafe-standards-deliver-promised-benefits