SIR – It would take Philip Bird (Letters, April 10) longer to walk one hundred metres than if he ran the same distance.

However he would use less energy, that is fuel, and this is because he is not compressing the air in front of him. Air resistance is very significant for cars as it’s proportional to the square of the speed.

If the speed is doubled, then the air resistance increases four times, and the engine has to use more fuel to work harder to push the car through. More fuel means more CO2, so it’s got very little to do with how long the journey takes.

He could also imagine how hard it is to walk against an 80mph wind rather than a gentle breeze.

Keith Thomson, Heights Lane, Bradford