The great thing about electric vehicles is that it makes it possible to greatly reduce emissions as the grid is transformed towards a larger share of reusable energy.
Another great trend is autonomous driving, which will enable a reduction in the number of cars by having cars as a service at a lower price. Fewer cars will mean that cars can be built better and last longer, as they can be more expensive. As long as these cars are electric and powered by renewable energy, the potential to reduce emissions compared with the current situation should be large.
[1] http://www.ucsusa.org/sites/default/files/attach/2015/11/Cle...
https://www.quora.com/Is-it-true-that-the-15-biggest-ships-i...
When you say they "left out the petrol", are you assuming that a better analysis would assume that the petrol is never made in the first place?
I mean, that might be the wish but we don't yet live in a world where the gas will stay in the ground if we don't use it to fuel one automobile at the margin.
If it's you as a consumer buying the next car to be produced, then it is entirely appropriate to compare it that way, because your single purchase will not change the entire life cycle.
Furthermore, according to the EPA, 27% of greenhouse gas emissions are for transport[2], however there's no breakdown by things like airplanes vs trains vs cars.
IMO, if people really want to 'change the world', we need to shift away from automobiles, consumerism, plastics, and conspicuous consumption. Live close to work, ride a bicycle, stop buying McMansions.
[1]: https://www.eia.gov/energyexplained/?page=us_energy_home
[2]: https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emis...