What is the mechanical reason behind leaving a car in gear?

logicalgal

If you leave a vehicle "parked" in NEUTRAL, your transmission is disconnected from the drive train and therefore your car is able to freewheel with gravitational pull (a slight grade or hill), and thus possible hit another vehicle or injure/kill a pedestrian, cyclist, skateboarder, etc.
When you shift to "Park" in an vehicle with an automatic transmission, the transmission is still engaged with the drive wheel and locked so your vehicle won't roll forward or backward.
When you shift into first gear in a manual transmission vehicle, the same thing happens.
This is a mechanical way of preventing your vehicle from unintentionally rolling into something while parked.
As an added note, using your parking brake BEFORE shifting into "PARK" or "FIRST GEAR", will also protect your transmission. Your vehicle will not then roll into the gear and damage your transmission over repeated parking times.
If a mechanic visits this forum, he/she may elaborate on this in more detail.
I hope that helps.

StanS

It makes it real hard for your car to roll away. The wheels would be trying to turn your transmission and your engine. When the engine is turning, the pistons are going up and down. Part of what makes the engine run is the piston coming up and compressing the fuel / air mixture in there. If the ignition is off, it's just compressing air which is difficult to do, making it hard for the car to roll.

Trying to get it to roll with it in first gear makes the engine try to turn faster (more cylinders compressing for any given distance) than any other gear.

Try this experiment. Park your car on level ground, without the parking brake on and in neutral. Go out back and try to push it. It's not too hard, even for a small person. You can even get a big car moving pretty good just by pushing it.

Now go put it in top gear, and do the same. It's difficult, but you can do it, even if you're a small person. Now put it in first. If you're really big, and put your back into it you can probably push it a bit, but it sure isn't easy.

They say leaving it in first makes it hardest for it to roll away cause it's the lowest gear, but it seems to me it'd be best to leave in in reverse. Reverse is lower than first, and if it wanted to roll away it'd have to get your engine turning backwards.

friedach

so it won't roll away if the brake fails. It does happen.

fwendt72

The engine will have compression built up in one or more of the cylinders. This compression makes it difficult for the engine to turn over. This in turn could help stop the car or slow the car should the parking brake fail.

Vorhees a jolly good fellow

Exactly what P1 says.

When you put the car into gear, and turn the motor off, the drivetrain will hold the car, if the handbrake fails.

The reasoning in it is that in order for the wheels to turn, they need to go through the diff, gearbox, and then turn the motor over.

Duct Tape

It stops it from rolling because the engine is not moving and the drivetrain is locked in place by the engine and clutch. That's also kinda why you can start a standard car by pushing it- because the clutch grips the drivetrain and turns over the engine.

StephenWeinstein

Because it keeps the engine and transmission connected to the wheels. Gravity is strong enough to pull a car in neutral, but not strong enough to back-drive the transmission and spin an engine that is off.

redhzkingswood

logicalgal has the right answer, but be warned that if someone runs into the back of your parked car and it is a manual left in gear, your insurance company may not pay for the repairs to the transmission.

UCANTCME

You will not lock the drive-train if you dont and then the vehicle will roll.

Joe

Also it should be noted that this should be done hand in hand with the drive wheels turned toward the curb or raised sidewalk if their is one; so the vech turns into the curb and will have to go up the curb... this will not happen and it will stop your car or truck.

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