Would putting my car in Sport and leaving the transmission in 8th gear at highway speeds (using cruise) be more fuel efficient than leaving it in drive with full auto? I just noticed that while in drive with full auto the car downshifts going up hill. If I put it in Sport and leave it in 8th the RPMs sits at 1600 no matter if it's uphill, downhill or flat.
Car: 2014 Chrysler 300S, 8 speed semi-auto transmission/Manumatic (which ever you prefer)
The primary reason to down shift is more power.
The 2 things effecting your gas mileage here are throttle position and RPM. As long as you can stay in 8th at less than half throttle, I would stay in 8th. If you need more than half throttle, I would downshift.
The power output of an engine is determined by how fast it can capture the energy released from fuel. There is an optimal range for that fuel-to-torque conversion, before and after which efficiency falls off. Whichever gear allows your engine to stay in its "zone" is the most efficient gear. If your car wants to shift, trust the engineers that built it.
Incidentally, this is why it's more efficient to "get to the point" when accelerating, getting to that zone of optimal torque more quickly (unless there's a red light in your immediate future). Smarter folks than I discuss the physics of it in this related post, from which I borrowed this image:
On flat ground, you can go by the general rule: the higher the gear the less fuel you burn. You also get less power, but it is more efficient, and it isn't bad for the engine provided it isn't lugging and you are above the idle speed.
When you're going uphill, however, it can instead be the most efficient to shift toward the point where you get the highest torque, particularly if your engine is running above 50% power output.
Engines generally deliver the best fuel economy between 1500 – 3000 RPM, depending on your engine. When you are running under load, the most efficient place to be running is at the peak efficiency point for your engine, which is usually somewhere in that range.
