Possible reasons for the drop in speed when the automatic transmission is engaged in reverse gear:
Electronics problems:
* Faulty gear selector position sensor
* Faulty reverse solenoid
Hydraulic problems:
* Low automatic transmission fluid level
* Clogged or faulty hydraulic control unit
* Faulty automatic transmission pump
Mechanical problems:
* Worn or dirty reverse bands or clutches
* Seized shaft bearing
* Damaged gear teeth
Engine problems:
*Low engine idle
*Weak ignition coil or spark plugs
* Air leaks in the intake manifold
Other possible reasons:
* Faulty flywheel or starter
*Wiring problems
Diagnostics:
1. Check the automatic transmission fluid level.
2. Scan the vehicle for fault codes.
3. Inspect the shift position sensor and reverse solenoid.
4. Check the pressure in the automatic transmission system.
5. Diagnose the engine for any problems.
Repair:
* Replace faulty sensors or solenoids.
* Flush or replace the hydraulic control unit.
* Check and repair the automatic transmission pump.
* Replace worn or dirty bands or clutches.
* Repair or replace a seized bearing.
* Replace damaged gear teeth.
* Adjust engine idle speed.
* Replace ignition coils or spark plugs.
* Eliminate air leaks.
* Replace faulty flywheel or starter.
* Repair or replace damaged wires.