What is the reason for the car shaking when accelerating at 40 km/h in third gear?

[SanJasmine]

The reasons for the car shaking when accelerating at 40 km/h in third gear are: 1. The front wheels of the vehicle have misaligned positioning angles, resulting in excessive toe-in and causing the vehicle to shake; 2. The tire pressure of the front tires is below the normal value, or the tires were not balanced after repair; 3. The wheel spokes of the front tires are deformed; 4. There are loose parts inside the transmission system; 5. The drive shaft is bent, causing dynamic imbalance and leading to vehicle shaking. Precautions for shifting gears include: 1. Do not look down while shifting gears to ensure driving safety; 2. Keep the steering wheel steady while shifting—hold the wheel firmly with one hand and shift gears with the other; 3. Avoid violently shaking the gear lever back and forth; 4. Ensure the gear and speed are properly matched.