From the EPA I get a US national average of 1,363 lbs CO2/MWh of electricity. Based on averages of 12,000 mile/year and 19.561 lbs C02/gallon and an average of 20.3 mpg I get the following:

440,000 MWh x 1,363 lbs/MWh = 299,860 tons of CO2 saved
299,860 tons/5.868 tons/car/year = 51,124 car/year

http://www.epa.gov/cleanenergy/energy-and-you/how-clean.html

Considering this excerpt;
“If the US switched just 20% of the decoration lights this Christmas season (to leds), we would save 440 GWh of electricity, or about 340 tons of carbon dioxide according to the EPA. That’s over 50 cars off the road for a year.”
Ok, I will argue ONLY the amount of “cars” this equates to…
It seems really odd that 440 GWh of juice would equate to ONLY 50 cars!

440 GWh = 440,000,000 kWh, if converted to Btu’ (x 3,414) = 1,502,160,000,000 Btu divided by, (I will assume) 113,000 Btu per gallon = 13,300,000 gallons per 50 “average” cars per year, / 50= 265,000 gallons per car… At that rate, each “car” would have to travel (at average 17 mpg) 4 and a half million miles per year!. Since the “average” is set at 12,000 mp year, I must / by 17 mpg = 705 gallons per car, really. Now, 265,000 gallons per car (from above encountered error) divided by 705 (about real number) = The above statement must be off by a factor of 375 times!
Thus 440 GWh = more than 18 THOUSAND cars off the road… ! And I thought it was going to be in the millions, oh well.
Now, if we can figure out how many gallons till the oil crunch… Better build that massive RE system now, or nuclear with breeders to save 99% of (otherwise wasted) non-renewable uranium!.