It depends on how long you are idling, or how fast you are going. If you know you are going to be idling for a long time, it's best just to put your car in park.
Need more info like what is the vehicle doing when it shakes, is it idling in park, driving down the road, braking, driving at highway speeds, etc.
It is more than likely a thermostat
theorically driving with the windows down will use more gas due to the disturbance to the car s aerodynamics
Many times when calculating miles per gallon, automotive companies will list both city mpg and highway mpg. City driving includes less miles, but more stopping and starting, and idling in traffic.
The spark plug gap can cause the RPMs to fluctuate, while idling. The more probable cause of the fluctuating RPMs is a bad camshaft.
More info will be needed to properly answer your question. There could be a multitude of reasons. Does your vehicle start at all? Does it stall while driving or idling?
The same on a car equipped with manual transmission. On an automatic equipped car it will use less idling.
More than likely a loose air hose. Easy to fix, just tighten or get a new jubilee clip
Yes, idling an engine puts more wear and tear on an engine (especially diesels) than normal driving does. It's also bad for the environment, not to mention just plain wasteful.
The cooling system is probably getting old ... but to leave a car idling for 15 min is not a good practice for the car or for your gas milage.
Slow down and leave more space between you and the vehicle in front of you.
you will proberley find the vehicle has either a misfire or air intake leak(petrol engine)/vacc pipe , i can only go as deep as the question appears, in a nut shell the question needs to be more in depth.