In some countries these days, vehicles must be sold with a carbon emissions rating. For those of us in countries without that requirement, or if you have an older vehicle, here's how you can estimate tailpipe carbon emissions based on your vehicle's fuel economy.

My calculations are based on the following formula taken from the U.S. Environmental Protection Agency (EPA) site.

CO2 emissions from a gallon of gasoline

= 2,421 grams x 0.99 x (44/12) = 8,788 grams = 8.8 kg/gallon = 19.4 pounds/gallon

Wait just a second - how can the carbon dioxide emissions from a gallon of gasoline weigh so much more than the gas itself?

The chemical reaction between the gasoline and oxygen in the air used in the combustion process creates carbon dioxide and other gases. Carbon dioxide, CO2, is one part carbon, 2 parts oxygen and oxygen also has a higher atomic weight than carbon - these two aspects makes the by-product of gasoline combustion heavier than the gasoline itself.

The following are few numbers based on miles per gallon/kilometres per litre, then broken down into carbon emissions per mile/kilometre.

Miles per gallon/Carbon emissions per mile (pounds):

10 mpg = 1.94 lbs
15 mpg = 1.29 lbs
20 mpg = 0.97 lbs
25 mpg = 0.78 lbs
30 mpg = 0.65 lbs
35 mpg = 0.55 lbs
40 mpg = 0.48 lbs
45 mpg = 0.43 lbs
50 mpg = 0.39 lbs
55 mpg = 0.36 lbs
60 mpg = 0.32 lbs

For folks in the world of metric:

Kilometres per litre/Carbon emissions per kilometre (kilograms)

2 km/l = 1.16 kg
3 km/l = .77 kg
4 km/l = .58 kg
5 km/l = .47 kg
6 km/l = .38 kg
7 km/l = .33 kg
8 km/l = .29 kg

So if you're a carbon offsetting sort of person and you want to figure out the carbon impact of your 40 mpg vehicle on a 100 mile trip, it would be at least 0.48 lbs x 100 = 48 lbs.

Bear in mind the figures above are conservative as they don't include other car exhaust gases, some of which are also greenhouse gases; the carbon impact of oil extraction, refinery and transport, the construction of the vehicle, the carbon impact of road infrastructure, etc. etc. etc. - this is just the combustion of the fuel itself, and just petrol/gasoline, not diesel.

If you would like to improve your fuel economy and in the process reduce carbon emissions (and save a few bucks), check out these gas saving tips.