>>costs less to run.
If it truly costs less to run, do you not think the livery companies wouldn’t be buying them already?
Not neccesarily. They may cost less to run, but then there is the capital cost of buying a new one to consider. No one is going to outlay that kind of cash when they have a perfectly working (if inefficient) vehicle already. This is why I added the original caveat “depending on how they go about it”. The solution is to phase them in - i.e. “from such and such a date, we only licence new vehicles that meet the efficiency criteria, but you can continue to operate older ones”. As time goes by, older cars get to the end of their lifespans and are slowly replaced with newer types. That way you get the efficency benefit with the minimum amount of pain.
A similar kind of process was done with computer monitors in the UK a few years back. Very different system of course, but similar process.