Diesel and CO2

Diesel engines are more fuel efficient than gasoline equivalents: it takes less Diesel fuel to drive any distance vs. a a comparable gasoline engine. However, Diesel engines produce more CO2 from burning Diesel fuel vs. a comparable gasoline engine. If you simply compare fuel consumption, Diesel engines appear to be much better than gasoline equivalents. However, if you compare CO2 emissions, Diesel engines, although still better than gasoline equivalents, are a lot closer.

All numbers in this post were obtained from fueleconomy.gov I try to compare equivalent engines as much as possible (i.e. similar engine volume and horse-power, on the same model car).

• 2014 Audi Q5
• 3.0 TDI: 27 MPG, 385 CO2 grams/mile
• 3.0 Gasoline: 21 MPG, 429 CO2 grams/mile
• If you drive 10000 miles in one year:
• Diesel will burn 22% (=21/27) fewer gallons of gas overall
• But Diesel will only produce 11% (=385/429) less CO2 overall
• 2014 VW Golf
• 2.0 TDI: 34 MPG, 297 CO2 grams/mile
• 2.5 Gasoline: 26 MPG, 342 CO2 grams/mile
• Diesel will burn 23% fewer gallons of gas, but produce 13% less CO2 overall
• 2014 VW Jetta SportWagen
• 2.0 TDI: 33 MPG, 310 CO2 grams/mile
• 2.5 Gasoline: 26 MPG, 342 CO2 grams/mile
• Diesel will burn 21% fewer gallons of gas, but produce 9% less CO2 overall
• 2014 VW Passat
• 2.0 TDI: 34 MPG, 290 CO2 grams/mile
• 2.5 Gasoline: 25 MPG, 350 CO2 grams/mile
• Diesel will burn 26% fewer gallons of gas, but produce 17% less CO2 overall
• 2014 Chevrolet Cruze
• 2.0 Diesel = 33 MPG, 307 CO2 grams/mile
• 1.8 Gasoline = 27 MPG, 333 CO2 grams/mile
• Diesel will burn 18% fewer gallons of gas, but produce 8% less CO2 overall

Diesel engines do reduce overall CO2 emissions. However, the gains are more modest that it appears from fuel consumption alone. Additional gains can be obtained from reducing the size of the engine, and the weight of the car.