I have a 2011 compass Latitude 2.0L 6 speed transmission and I have an odd question.
Does the automatic transmission dislike 6th gear? It seems to not want to use the gear unless I manually move into the gear and if I do at a constant speed my rpm drop like a rock and my fuel economy goes higher.
Does the transmission need a certain speed in MPH/KPH to go into 6th or why would it avoid using it when I think it should?
I dont know the specs of that particular transmission but any gear on a transmission requires the car to be at a certain speed to be useful. If you shift to 6th below a certain speed threshold, the rpm will drop to a point where the engine cannot produce enough power or torque to maintain that given speed, so the engine will start to stall. If you were to continue this way on manual transmission, the car will slowly lose speed, you would either need to shift down, or you will slowdown so much that the engine will actually stop since it cannot maintained an RPM even above idle. On a modern AT, the engine would not stop as the torque converter would simply not be able to rotate the clutch on the transmission and you engine would effectively be in natural.
The reason why your MPG drop is also this. At a lower than recommended speed for 6th gear, you would need to press the throttle pedal more, so that the engine produces more power in order to not stall and slow down. So say, if you were giving 30% throttle at 5th gear to maintain 55MPH, at 6th gear, you would need to give 45% to keep the same speed. I made up the numbers as an example, please dont focus on those. The point is, it will cause the car to use more gas.
The same logic also applies to why you need to downshift when going up hill.
Also, weaker the engine, more prominent this effect will be, as a powerful engine can still generate enough power and torque at low RPMs to maintain speed. Turbo engines are particularly prone to losing power this way, as at lower RPMs turbo cannot spin fast enough and cant generate enough turbo boost, so the engine loses even more power. It is also the same reason why turbo engines do a terrible job at matching their "official" MPG values. In real world driving conditions, it is quiet difficult to keep an turbo engine at an RPM low enough to get that listed MPG value while not stalling the engine.
A quick goggle search indicates that most ATs wont shift to 6th gear(even with driver input) under 40mph and it wont improve MPG unless you are above 70-80MPH.