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Q: I changed the gearbox but the car is only moving in reverse and it needs acceleration up to 6000rpm to reach 30mph.

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I was having harsh shifts with my old gearbox it was not engaged in reverse at all,so i bought a second hand one. This one have very smooth reverse but on D it will move only 25kmph even if the engine is at it maximum i.e.8k rpm. Is the gearbox not good or is there any sensor problems. Plz help.

My car has 140000 miles.
My car has an automatic transmission.

It sounds like the transmission is slipping. It is not good to max out the RPMs to make the vehicle move. Check the tranmission fluid level and the condition of the fluid. If the fluid is black, then I recommend rebuilding or replacing the transmission if it will not work properly. If the fluid is normal color or varnished, then the torque converter is broken. If you need further assistance diagnosing your vehicle, then seek out a professional, such as one from YourMechanic, to help you with your gear slipping issue.

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