I tried this one day a couple years ago. While on a road trip for work, I stopped at a gas station for lunch. Here's what I did:
Filled up.
PUlled around to the parking and ate lunch and took a nap for 1/2 hour with engine running (A/C on, it was summer).
Realized the fuel had gone down noticably, so filled up again and hit the road.
1/2 hour later happened to see another gas station. Even though needle hadn't moved much, filled up out of curiosity.
Result: I burned more fuel in 30mins of idling than I did in 30 mins of cruising.

That's not uncommon. Idling under load is the hardest thing your motor ever has to do. And it shows in fuel consumtion.

Ask a cop how much fuel THEY burn in one shift....

That's not uncommon. Idling under load is the hardest thing your motor ever has to do. And it shows in fuel consumtion.

Ask a cop how much fuel THEY burn in one shift....
what he said ^^^^