What are the pros and cons for the two possible "gear directions" when parking the car on a hill?

Question

Let's suppose I leave my car in gear when parking on a hill (besides using handbrake and turning the wheels towards/away from the curb, depending on the direction I'm parking in). In case that matters, let's suppose the motor is gasoline (but I am also interested in answers for diesel-motors).

By the two possible "gear directions" (is there a better term for this?) I mean the following:

1. Gears in the same direction as gravity, i.e. if I park pointing downhill, then first gear, if uphill, then reverse.

2. Gears against gravity, i.e. if I park pointing downhill, then reverse; otherwise, if uphill, then first gear.

I did some research, to find which one you should do, but found there is no definitive answer. This is what I found:

• If the gears are "against gravity" (i.e. reverse downhill, first gear uphill), then if the car accidentally moves, then the motor can be damaged by moving "backwards". (If I understood correctly, the reason is that the drive belt only works in the forward direction, and thus valves will hit against pistons.)
• If the gears are "in the same direction as gravity" (reverse uphill, first gear downhill), then if the car accidentally moves, the motor can jump start alone, and the car will move forward autonomously from that point.
• Newer cars cannot jump start automatically in the above case (only older cars).

My questions are the following:

• Are these conclusions correct?
• Are there any other pros/cons for the two directions?
• How are newer cars immune to being automatically jump started, if they move in the same direction as the engaged gear?
• How can I check if my car is new enough to not be affected by this problem?

PS: I'm already aware of this other question, which has been closed as opinion based, but I think that mine (1) isn't a duplicate, and (2) is not opinion based, because I am asking more concrete questions.

PPS: I also know that there are arguments against leaving the car in gear at all, when parking (some of them also mentioned in the question above), but they are out of scope for this question.

Answer 1

You've got several things wrong, as you've made a lot of assumptions.

First, in an engine if the ignition isn't turned on (ie: if it doesn't have power), you cannot push start it. You can turn that engine all day long and it isn't going to start. Just won't happen. It takes fire to make it run, which in this case is a spark (we're not talking about diesel engines here, just gasoline). If you don't have the key on, or haven't gone through many different hurdles, it's not going to start. In this case, it doesn't matter if the car is newer or older ... well, I guess if it was a lot older it might make a difference, but nothing from probably the 1930's forward. Bottom line on this is, your car will not spontaneously just start because it is moved with it in gear. Period.

Second, the timing belt/chain works in either direction. It will turn the timing components whether the engine is going backwards or forwards. I will say, it probably isn't good to run your engine in reverse, but it won't kill it. The only reason it wouldn't really be good for it is because the oil pump only works in one direction. Instead of providing more oil to the engine components which need it, you're actually purging whatever oil is already in there. Sucking it dry, as it were.

As for the rest of it, I'm sure there are reasons some would say leave your transmission one way or the other. I'm not going to answer that because it is conjecture ... IOW: this is the pure opinion part of the question. Some will say it is best to do it one way, while another will tell you it's better to do it the other. Both probably have valid points. If it were me, I'd ensure it's left in gear, one way or the other. Sometimes the parking brake may just not be enough. The main thing to do when parking on a hill is to ensure your tires are turned the correct way at the curb: If pointing uphill, point your tires away from the curb; if pointing downhill, point your tires towards the curb. This will help ensure your if your vehicle does start to move, the curb will catch it. If there isn't a curb and you aren't sure of your parking brake ... don't park on a hill.

EDIT: I realized I didn't answer the other part of your question ... diesel engines ...

As far as a diesel engine goes, it runs off of compression. That is its spark as it were. Generically, all a diesel engine needs to run is the fuel and air, plus the motion of the engine. Theoretically, it could run without the ignition on, as long as it has some fuel in order to do so. A diesel engine can run off of pretty much any oil, including engine oil (yah, the stuff in its crankcase). That said, though, as long as a diesel engine is not leaking oil into its intake runners, it isn't going to run - there's nothing there to combust. Even on older diesel engines, the way you kill them is to turn off its fuel source. This could be through a mechanical means (ie: shut off valve actuated via a manual device, like a pull), or electromechanical valve (ie: shut off valve actuated via a solenoid). Either way, as long as these devices are in order, it will not start as the engine goes around. Again, not going to happen.

Answer 2

The gear ratios in most manual transmissions are such that first gear has a lower ratio than reverse (actually a higher numerical ratio is a better way to express this but I'll follow the convention of this discussion). Often the reverse ratio is about halfway between first and second gear. I guess the reasoning is to allow a bit faster speed when backing up a long way.

Using the lowest ratio gear is the important point so I've always used first gear for parking. The issue about the engine being run backwards is a good one but I would expect manufacturers to design for this. Perhaps a high-compression high valve-lift race motor may not be immune but I haven't had the privilege of driving one on the street :-). Even in that case the race motors I've seen up close the chain or timing belt has very little slop.

Answer 3

The reasoning behind leaving in gear is that first and reverse gear are both low ratio gears, this provides enough resistance against the car moving should the brakes fail. If you consider how engine braking works to slow a vehicle and how quickly, you will see that using low ratio gears is also an effective brake method to prevent movement from standstill.

Answer 4

The top (most voted) answer draws pretty good conclusions. But I would like to add regarding the Forward vs. Reverse argument; that (with the exception of older, or larger trucks) that reverse is almost always lower than First. They nearly always share the same shaft in the transmission. One exception that I can add with great certainty, is my 1964 Chev C20 pickup truck. It was purchased without the (optional) emergency brake. I know from experience that 1st gear will hold it on 40° incline (driveway) without the aid of a curb for several days, and not move.

Lastly; if the car/truck has low/poor compression. Neither gear will matter, and given that diesels almost always require higher compression than gas powered cars. Diesels will more likely carry an advantage over their gas powered counterpart(s).

Answer 5

I haven't driven a stick in years, but as I recall Reverse always seemed a little less certain/sticky than first. It was going against a spring and seemed very easy to pop out.