LTC8K6
Lifer
That is usually because the gear ratios on the Autos are taller than the Manuals.
For example the 2012 Mazda 3
Auto:
First Gear Ratio 🙂1): 3.55
Second Gear Ratio 🙂1): 2.02
Third Gear Ratio 🙂1): 1.45
Fourth Gear Ratio 🙂1): 1.00
Fifth Gear Ratio 🙂1): 0.70
Sixth Gear Ratio 🙂1): 0.59
Final Drive Axle Ratio 🙂1): 3.89
Manual:
First Gear Ratio 🙂1): 3.21
Second Gear Ratio 🙂1): 1.91
Third Gear Ratio 🙂1): 1.36
Fourth Gear Ratio 🙂1): 1.02
Fifth Gear Ratio 🙂1): 0.94
Sixth Gear Ratio 🙂1): 0.79
Reverse Ratio 🙂1): 3.45
Final Drive Axle Ratio 🙂1): 4.18/3.52
Sporty drivers get better acceleration from the manual with a slight hit to Fuel Economy, whereas normal drivers get the extra mpg. If you equip the manual with the exact same gear ratios as a conventional auto transmission you will be better fuel economy from the manual. Mechanical linkages are always superior in terms of efficiency.
Obviously dual clutch transmissions being mechanical in nature as well, are exempt from this logic.
The Autos can tolerate the higher ratios...the manuals generally cannot.