This topic comes up so often - and yet is so misunderstood.
Most of the world still drives a manual clutch transmission by choice, not because of cost. Why? Driving performance, response, and fuel efficiency where fuel is generally much more than the USA and in Europe driving fun is more important than just off the line acceleration.
Why are automatics (AT-TC, CVT, Automated clutch, DCT) even close today in EPA MPG ratings to manuals?
1. Programming to game the test for the drive cycle- real world results are worse
2. Programming that gives early torque at the expense of mid range driving response - feels fast initially then nothing
3. Lazy ratios that trade off performance significantly vs. mileage
4. More gears locking up in torque converters
5. In the case of automated manuals, they have the benefit of manual clutch internals and their direct drive efficiency with electronic clutch shifting
6. CVTs are more efficient in lower speed than torque converters, but lossy at mid to higher speeds
Why do a few manual transmissions (stick/clutch) get worse US EPA mileage than their AT-TC or CVT counterparts or even DCTs?
1. Gear/final drive (differential) Mainly the gear ratios have much more acceleration vs. the automatic version. As much as 2.5 sec 0-60 improvement for a 2 EPA MPG penalty.
Despite this, real numbers from fuel economy.gov often show better real world mileage with performance advantage for manuals vs. automatics..
2. Test programming Some automatics, as mentioned, have very lazy shifts for the US EPA test cycle. This is also common with Japan market cars that have to meet their 10-15 drive cycle that in real world results can drop MPG figures by double digits depending on the car. If the test cycle short shifts the manual to make up a more aggressive gear, better MPG will result. They don’t test on performance exactly. They test on speed, which requires an understanding of gear ratio as a test driver.
Automatics generally don’t do as well in the European drive cycle as it is most like real world driving.
3.Driver ability: many US driver no longer understand the concept of driving in the proper gear with torque and momentum. They have not had to understand the power band relative to gear ratio and wheel grip. This is common knowledge in Europe.
Why are manual transmissions more efficient and have more control even today?
1. Driver involvement with foresight An automatic is a reactive device. It only reacts to what it has at the given moment. It cannot see ahead. A manual driver has to plan ahead to braking, cornering, hills, traffic, and plan the gear needed ahead of time. This is why racing cars and most big rig trucks are manual clutch transmissions for both performance and efficiency.. They may have electronic clutches or paddles or ‘stackable’, but the driver is responsible for momentum and torque at the wheels to keep the vehicle in control. This works both ways – a lousy driver will rough and inefficient with a MT, but also with an automatic. How inefficient with the AT depends on the mismatch of the gear ratio and momentum with desired speed, hence the potential variance could be wider. A good example is a truck with an AT and a trailer driven by a poor driver up in mixed speed driving. One needs a large amount of torque to offset the lack of foresight and proper gear ratio with such heavy load behind/.
2. Direct drive A clutch in a manual is always connected and not lossy or slipping. While programming and lock up clutches have improved automatic efficiency. Automatics with torque converters tend to be less efficient is city driving or mid-speed suburban driving cycles due to the generally pumping losses plus the converter may not be locked up a good portion of the time. CVTs tend to be the opposite – doing well in mixed throttle speeds, but not as good at higher steady speeds.
3. Gear ratios While most engines operate best with 5 gear ratios and a cruiser gear (6th), there some applications where a 7th gear is useful, but is generally for very high speed racing on road courses (Formula 1, etc.) or heavy trucks. Automatics with multi-speeds (7-9 gears) are simply splitting the partial power bands to offset the lag creating by being in the wrong gear at a given time. They are not more efficient, simply offsetting fundamental efficiencies with frequent shifting in part throttle.
4. Car control and safety Go to any performance driving school (Skip Barber, Bob Bondurant, etc.) and knowing what gear to be in and how much torque to apply to the road is the fundamentals of car control. This requires manual control, foresight, and direct drive. Knowing when to shift is fundamental, even in an automatic.
5. Driving isn’t linear Comparing each on a drag strip or block to block isn’t realistic. Cars need to be run in traffic, up hills, mixed speeds, hard corners, trailering, etc.
6. Simple internals Does not apply to every gearbox, but most. Most manuals are simple compared to a torque converter or a CVT. Parasitic losses are the minimal benchmark. Some act effectively as pumps.
Bottom line is physics. Automatics don’t do anything better vs. a manual with skilled driver. They don’t create efficiency over a manual.
They aren’t more efficient, apples to apples, they have less driver control and safety, and less response and performance.
They simple exist for convenience. Given the amount of urban traffic, not surprising why they are popular.
Me? I only drive an automatic as a rental. Because I have no choice, except in Europe.
