Is it more efficient to let your car idle in neutral for a short period of time (say you are running into a store real quick) or to turn it off and then restart it 5-10 minutes later?
Turn it off.
Yes, turn it off. I found these two pages that state it only takes a minute or two’s worth of gas to start it up again. So unless you can do the entire in/purchase/out store routine in 60 sec flat, you’re not saving anything - plus giving the opportunistic criminals their first (or tenth) big break.
I don’t know how many times I’ve seen very nice cars idling away in a big parking lot 50 feet away from the store while the owner is on a “short trip”. If I were so inclined I could have stolen a car (having no expertise in burglary whatsoever) every week or two from right across the street… not enough to live off but plenty of crack money :D.