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20. Do you need to quit using A/C?
Not really. If you drive at highway speeds with your windows open, aerodynamic drag will consume more gas than A/C. At lower speeds you may want to open your windows and turn the compressor off, at higher speeds, use the A/C. It's time to close the windows at 50-55 mph for most cars.

21. Should you avoid excessive idling?
Yes, but that doesn't mean you should turn off your car at a red light or when coasting in neutral. Such solutions are unsafe, and you'll consume more gas when you start your engine back up. But do try to avoid parking or idling for any prolonged period with your engine on. Remember that your engine gives you 0 MPG when idling, so when it's running it's costing you money. On the other hand, remember that starting your engine consumes the same amount of fuel as idling for a minute or so and it also puts an extra strain on your battery, starter, and ignition switch, reducing their life and leading to their premature replacement - which will surely cost you money. You just need to apply common sense here.

22. Do you need to use cruise control?
Speed control works best on straight roads. If the road you're on has hills, you lose. Experienced drivers will disengage the control, accelerate down the hill and decelerate up the hill. Cruise control will try to maintain the speed, loosing inertia down the hill and guzzling gas up the hill.

23. Should you stop warming up your car on cold mornings and start driving right away?
It depends... a warm up consumes an enormous amount of gas. You won't hurt the vehicle by driving right away... but you will be cold till the heat is working. So this is sort of a climate dependent answer, if you're in Alaska, and the temperature is -20F *inside* your car, by all means let it warm up first.