Calculating a car’s CO2 emissions from its mpg

Motorists who drive cars that were first registered before 2001 are unlikely to be aware of how much CO2 their vehicle emits.

A car’s V5 document (sometimes referred to as the logbook) details the amount of CO2 the vehicle emits per kilometre, but this is not the case for vehicles registered before 1st march 2001.

How to calculate CO2 g/km from mpg

If you know a car’s mpg figure, you can use a simple calculation to work out its CO2 g/km figure.

The calculation is simple, but you may find it helps to use a calculator.

How to calculate your cars CO2 emisions Knowing only its mpg figure (in this example the car does 40mpg)
For petrol engines divide 6760 by 40 (the car’s MPG) 169
For diesel engines divide 7440 by 40 (the car’s MPG) 186
A petrol-engined car that does 40mpg has CO2 emissions of 169g/km
A diesel-engined car that does 40mpg has CO2 emissions of 186g/km

A spokesperson for the Environmental Transport Association (ETA) said: “Now that road tax is calculated on the basis of CO2 emissions, an increasing number of people know how their car emits, but this may not be the case for the owners of older cars.”


  1. Richard


    The saddest issue with British motorists is that they have been allowed to become totally ignorant about the whole concept of fuel economy. Most motorists are unaware that CO2 emissions are directly related to fuel consumption but this is of no consequence as the CO2 number tells them nothing about how they drive. As for fuel consumption, motorists have been kept entrenched in MPG, a figure that is completely irrelevant when fuel has been sold in Ltrs for over 20 years. Few know how far their car will travel on a Ltr of fuel. If standards were to be updated so that fuel consumption figures were to be quoted in Miles per Litre or Litres per 100 Miles, then motorist might just begin to think again about the effects they can have by driving economically.

  2. Miles


    Where is the formula that describes how to use CO% emissions (from an MOT emissions test) to get the CO2 g/km ? At the moment the CO% is measured and petrol volume flow rate is extracted to arrive at a CO2g/km figure. Okay but where is the formula so that people can check a mistake has not been made?

  3. no


    is this real?

  4. David


    where do your 6760 and 7440 factors come from? are they just for combustion of the fuel or do they include the CO2 emissions from refining and transporting the fuel?

  5. Frank Thurston


    Still cannot see how these co2 values are arrived at. A new BMW with 3ltr engine supposedly has 42 mpg/142 co2. If you work that on the calculation of 7440/42mpg you get 177 NOT 142! My 2011 3ltr diesel I get an average of 44-45 mpg. I have had as high as 54 mpg. Working on that same calculation my car mpg of 45, is 165 co2. I am paying tax on 177. Is this another screwing the government and car manufacturers are kicking us with?

  6. Glenn Atterton


    Is there a formula for two stroke motorcycles?

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