It’s by design for automatic transmissions. When in Drive, if you’re not pressing the brake pedal the usual action a driver will take is to accelerate, so the car engages the clutch, which in turn connects the engine to the wheels. Since the engine is running and the car keeps it revved at around 700rpm, the car moves.

Newton’s second law of motion describes the relationship between force and acceleration. They are directly proportional. If you increase the force applied to an object, the acceleration of that object increases by the same factor. In short, force equals mass times acceleration.

The idle speed of cars would be around 1000 rpm, this is to prevent engine from stalling. At this speed, the car still produces some power so obviously when the clutch is released (in 1st gear), the car will move forward. The car might move faster in 2nd and 3rd gears without use of throttle, for the same reason.

Transmission slipping, or gears slipping, is a common transmission problem and a real cause for concern. If your engine revs up as you press down on the accelerator but your vehicle doesn’t accelerate as per usual or as you would expect, it’s a troublesome sign that can lead to a full-blown transmission failure.

All vehicles in gear will move whether you apply gas or not. Your engine is at idle, which does not equal off, it means the engine is running at 600–1000 rpms (depending on vehicle).

Seven Signs Your Transmission Needs to be Serviced

1. 1) Your Transmission is Slipping.
2. 2) Your Transmission is Leaking Fluid.
3. 3) Your Transmission Emits an Unusual Odor.
4. 4) Your Transmission Causes Rough Idling.
5. 5) Your Transmission Shifts Poorly.
6. 6) Your Vehicle Accelerates Poorly.
7. 7) Your Check Engine or Transmission Warning Light is On.

Transmission fluid does need to be changed from time to time, and the most accurate answer to the question is that the fluid will usually last between 30,000 and 60,000 miles. That said, if your vehicle sustains heavy use, you could have to change the transmission fluid in as few as 15,000 miles.

Pull out the transmission fluid dipstick. Wipe the dipstick on a clean rag or paper towel, reinsert it and pull it out again to check the transmission fluid level. The fluid level should be between two marks labeled either “Full” and “Add” or “Hot” and “Cold.” Usually, you should not have to add transmission fluid.

Let it run for about 5 minutes so that it can warm up. Some car manufacturers will recommend you turn the engine off before checking the transmission fluid, but most don’t recommend this.

Wondering when to change your automatic transmission fluid (ATF)? Most manufacturers recommend replacing transmission fluid every 30,000 to 60,000 miles. If you tow, or do mostly city driving with constant changes of gears, it may be wise to change your ATF even more often.