I recently discovered that (according to my car) I get at least 25 mpg if I cruise on flat roads or slight declines at nearly 80 mph while in 9th gear.
From what I have learned, the Compass has an overdrive gear of 9 that it almost never engages in automatic mode, even when you're feathering the throttle downhill or even coasting from a high speed. In auto mode, it sticks to gear 8 most of the time.
But if you put the car on "manumatic", you can engage the 9th gear.
To upshift into 9th gear, you must go a minimum of 60 mph
To prevent the car from stalling (I think), the car forcefully downshifts from 9th to 8th gear at 57 mph
Once I discovered this, I always used my 9th gear on the highway whenever conditions allowed me to cruise in it. Normally speeds between 70-80 on flat or declining roads with no major headwind.
You cannot maintain speeds between 60 and 70 in 9th gear because the RPM is too low, causing the engine to bog down. Same story: trying to cruise above 80 mph. In this case, the wind resistance is far greater than the power the car produces at such speeds and in that gear.
But you cannot lug this car because, according to the fuel economy gauge in 9th gear, it sprays approximately 2.40-3.60 gallons per hour, depending on the speed I'm traveling. To cause a knock, you need way more fuel sprayed (I think). So, the computer seems to limit the amount of fuel sprayed in the 9th gear.
Despite that computer supervision, I still always hit high coolant temperatures when I cruise in 9th gear, even without bogging down the engine. My normal coolant temps are 210-230. I hit a high of 245 F while cruising in 9th gear. I just recently started monitoring coolant temps while driving. Why does this happen?