What kind of damage/trouble might this practice cause? Will this ruin your transmission? Do no harm whatsoever? Something in between.
On a modern car the only harm it does is to add some wear to the linkage, and pthe engine if you keep it in a lower gear than it would have been had it shifted itself.
Shifting from one forward gear to the next up or down will do no harm. For instance when you are going up or down a mountain it would be good to shift to the next lower gear going up (to get more power) and also to the next lower gear going down to use the engine braking. Just don’t do this all the time as some poster’s self-styled Formula 1 husband was used to doing.
What you do not ever want to do (any many do it) is to shift from drive to reverse (and vice-versa)while the car is still moving. This can quickly wreck a transmission.
you also do not want to be in Neutral at a stop light, then when the light turns green you press on the gas pedal before shifting…
I think I wasn’t totally clear. I meant shifting out of drive into neutral while the car was moving (like when going downhill because it decreases gas usage/increases mileage) then shifting from neutral back into drive, still while moving.
Sometimes when performing that action there would be a clunk sound and the car would lurch a little. Now the car has a clunk sound and lurches when shifting from park into drive (through neutral) when standing still. I’m wondering if the two are related…
I meant shifting out of drive into neutral while the car was
moving (like when going downhill because it decreases gas
usage/increases mileage)
Doing that may be causing you to use more gas, not less. Many cars now shut the injectors off during deceleration. If you slip the transmission into neutral, then the injectors need to keep pumping gas to keep the engine running.
Coasting in neutral is also against the law in many states, and potentially dangerous.
You have a computer telling the transmission what to do and how to do it. Don’t trust the SOB so much.
Why would you want to? The difference in fuel usage is likely to be minimal and if you have a a modern car, it likely will cause your car to use more fuel, not less. In gear most modern cars shut off the fuel totally when in gear and the engine is being pushed over idle speed.
Don’t bother.