I'm not saying one shouldn't try to conserve, I'm just saying that it's hard to know or measure the results. ;-)

Not idling here is a 1/2 year luxury. From May-Oct it's usually too hot to turn off the car and let the car and occupants heat up.

Here's a constructive tip (since I seemed rather pessimistic):

The AC compressor takes a lot of power to drive when engaged and one's foot has to increase the revs to compensate for power loss. As a result the AC is the largest fuel consumer.

Switching off the ac after one is cool enough saves a lot of gasoline. So it also make sense to tint the windows, preventing heat transfer from the sun, allowing the AC to work faster and allow the interior to remain cooler longer after it has been switched off.

Tire inflation is one factor but tire circumference is even more important. A thinner but larger tire that gets more "rotation" out of each axle revolution will save more gas.

So if one has tires with an aspect ratio (percentage of the tire sidewall to the width of the tire) of 50 or 60 changing to a tire with an aspect ratio of 70, and the bigger circumference it creates, will go farther on the gas per axle rotation. 235-70 R15 tires would be better than 235-60 R15 for gas mileage. Keeping the width in mm down would be even easier on gas (less friction) so one wants to blend width and circumference to favor economy. A 225-80 R15 would in theory roughly keep the desired circumference and yet narrow the tires for less friction with the road.

Of course this affects handling, and raises the car a bit more off the ground increasing the center of gravity, but should be good for at least another 1 mpg.

Not a huge difference but these days every drop counts.

;-)