Even if you assume that a tug has only 1/10th of the fuel efficiency of an average SUV there is no way it could burn a couple hundred kgs for 3-5kms of push to the runway.
We use single-engine-taxi in and out whenever there is a longer taxi time and there is no evidence of increased engine stress if the required warm-up times before departure are adhered to.
Towing to the runway and no-diesel ground power won't save the planet single-handedly but it sure makes sense to cut emissions and noise wherever possible.
It would, of course, save a lot more if we just got startup and pushback clearances in time to avoid endless holdings before take-off. It is pretty inconceivable that we're unable to predict at least a little better when to release A/C from the gate in order to have not more than 2-3 waiting at the holding point.
It would save even more if we'd invented a better way of sequencing A/C so as not to have dozens of them in holdings (left of the channel) or flying useless mileage along huge waypoint-based traffic patterns (right of the channel) at inefficient speeds and altitudes.
AEA claims that we could save app. 10% of all fuel burned in aviation by just having a single Eurocontrol ATC and that is probably not far off the mark.
All these better potential savings are no excuse for not doing the small stuff and sometimes taking public tranport ourselves
