Why does the engine speed increase but the car speed doesn't when stepping on the accelerator?

[Harley]

When the engine speed increases but the car speed doesn't when stepping on the accelerator, it may be caused by slipping due to worn clutch plates. Here are specific details about engine speed: 1. 800/min1200/min: During cold starts, if the idle speed is below 800/min or above 1200/min, it indicates poor engine condition. 2. Below 2000r/min: Try to avoid letting the engine operate below 2000r/min while driving. If the speed is too low, the engine will lack power, and the excessive load at low speeds can damage the engine and easily lead to carbon buildup. 3. Above 4000r/min: When the engine operates above 4000r/min, the combustion time shortens, leading to incomplete fuel combustion and a significant increase in fuel consumption. If driving normally in fifth gear, the speed usually reaches 150km/h, which is already a relatively dangerous speed. 4. 22003500r/min: The engine operates most economically and performs best within the 2200~3500r/min range. It is advisable to maintain the engine speed within this range during regular use.

[StHarper]

I've been driving for over twenty years and have encountered this situation where the engine revs soar when the accelerator is pressed but the car doesn't speed up several times. The most common issue is with manual transmissions where the clutch disc is worn out, and the power simply doesn't reach the wheels. Last week, I worked on an old Jetta where the owner complained that the tachometer hit 5000 rpm while climbing a hill, but the car was only going 40 km/h. Upon inspection, the clutch disc was worn down to the metal. For automatic transmissions, it could be a failure of the torque converter lock-up clutch or a stuck valve body. Another time, I came across a modified car with a big turbo installed but without proper ECU tuning, so the turbo pressure couldn't build up. Also, dragging brakes are a hidden culprit—if the calipers don't retract properly, it's like driving with the brakes on. I recommend checking if the brake discs are abnormally hot.

[Clifford]

My SUV had this issue last year, where the engine roared when stepping on the gas but the speed crawled like a snail. The auto repair shop said the transmission fluid hadn't been changed for eight years and had turned into sludge, causing the shift solenoid valve to stick. At the time, the mechanic used a diagnostic tool to read the data stream and found the actual gear was stuck in second. Another easily overlooked point was that the catalytic converter was clogged, preventing exhaust from escaping and making the engine struggle. I remember a friend's car had this problem after filling up with cheap gasoline, and later when they opened the catalytic converter, they found the honeycomb ceramic was completely blocked into a solid brick. Now, whenever I go for maintenance, I always specifically remind them to check the transmission fluid condition.

[LaAllie]

Friends who are into car modifications should pay special attention, as improper ECU tuning can lead to such issues. A street racer I know adjusted the turbo pressure to 1.8Bar, causing the stock clutch to slip under the strain. Additionally, modified intake and exhaust systems may result in excessive exhaust backpressure, making the engine feel strangled at high RPMs. Those who install oversized wheels without adjusting the speed ratio face a hilarious situation—the speedometer shows 160 while the actual speed is only 130. At a track day, I once saw a heavily modified Fit with overly close-ratio final drive gears, causing severe torque loss at the wheels in fifth gear. These chain reactions from modifications often require a complete system recalibration.