Cars require more energy to be built than they use in their lifetime. To reduce overall energy consumption and thus carbon emmisions we should use vehicles for as long as possible.
Is this actually the case? :-?
Ive been told that a Vauxhall garage near me has a government scheme thing at the moment that if you buy a brand new car, you get £1000 back for ANY car which you trade in...
The down side of this is the cars which are taken in part-x HAVE to be scrapped! 
And there’s some STUPID people out there... Apparently, someone is buying a brand new Insignia I think, and their trading in 2003 plate TD Vectra with 80k on the clock... So they will get a grand for it, and a 2003 plate car has to be scrapped? How’s that good for the environment?
There’s an immaculate Y reg Astra 1.8 Parked up, been told it looked mint, very clean inside and out.... You've guessed it, its off to the sky too! 
Yes that is true, if you consider all the energy required to dig the ore out of the groud, refine and smelt that ore into a material, transport it to the factory, cast,stamp or manufacture that material into a usable part, transport parts to assembly plant, assemble the parts into car, transport car to showroom.
Then after all this someone running an old car untill it can no longer be repaired is far more enviromentally friendly.
Add to this the scrapping of a car, if all metal then not to bad, but these hybrid cars are the worst possible offenders when it come to recycling, the batteres and motors are awfull to dismantle..
The real climate criminals are people in new cars I am afraid, regardless of fuel used.
Mike