I drive a manual transmission car and I find my mileage to be comparable if not lower to an automatic transmission car. While activating cruise control, my mileage rises by a few kms because of this I have always wondered which transmission for my next car would be the best, considering mileage to be an important factor.
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If the actual transmission technology and engine is the same, then automatic transmissions always win the efficiency game, unless the software used is ridiculously poor or you’re driving in extraordinary terrain.
The reason is simple: Automatic transmissions will keep your engine in the sweet spot of revolutions per minute, where power vs. consumption is optimal. If you’re getting slower than your current gear can support optimally, the transmission will automatically shift down, and vice-versa. In a manual transmission car, the driver is controlling all of these decisions. Even if the driver is considering these aspects (which they rarely are), they would still need to actually do the shifting of gears at the perfect times every time to beat the automatic transmission. And most drivers are too lazy for that, or they can’t shift gears at the perfect time because they are in the middle of a turn and need both hands at the wheels, or they have the radio so loud that they don’t realize the revolutions per minute are too high etc. Add to that, that the ordinary driver can’t shift gears nearly as fast and flawlessly as a modern automatic transmission can.
The only advantage a manual transmission car has in terms of mileage (apart from a little bit of weight reduction) is that the driver can anticipate the near future, while the automatic transmission can’t (yet). So if the driver knows that more power will be needed shortly, they can shift down before accelerating, which saves a little bit of fuel. And if the car is going downhills or will have to break soon, the driver can use the shift-down to add some breaking power without sacrificing fuel to idling.
So overall, unless you compare cars from the 1980’s or you’re regularily driving in extreme terrain, automatic transmissions are definitely more fuel efficient.
Edit: That being said, I’m a huge fan of manual transmissions and I have been driving manual shifts almost exclusively for 15 years. I have now switched to automatic transmissions and find it so relaxing, even if the loss of control can be felt at times, especially when maneuvering in narrow spaces or at steep hills.
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