Awl --
There was a brief ad campaign in NYC, to the effect that letting your car idle more than **10 secs** wastes gas.
Iow, the "cost" of restarting your car is equivalent to only about 10 secs worth of gas.
NYC has pretty strict idling laws, which, iirc, limit the legal idling to 3 minutes. I guess that could be whittled down to 30 sec!
fwiw.
From a practical pov, proly the biggest gas-waster is under-inflated tires, for a given car. And speeding.