I noticed on a vehicle years ago with a automatic overdrive transmission which will dramatically lower engine rpm to about 2000 to 2200 rpm at 60 to 65 mph at a true longer distance cruising speed outside the outer belt in rural areas.
I read these on a C body Coupe DeVille Cadillac with both (BCM) body control module and (ECM) engine control module engineered with built in on-board diagnostics with a FWD THM440 xmission and HT4100 V8 transverse engine. The year is not important.
Before the days of Interstates, the best fuel economy was at about 35 mph due to urban and rural driving habits at $ .12 to $ .17 per gallon.
Very few vehicles had overdrive transmissions, a Uncle once stated a 82 hp. 1939 Plymouth Coupe during WWII he owned got 20 to 21 mpg driving from Columbus OH to Chanute Field in "Illinois" at 50 to 55 mph. This Plymouth Coupe had a flathead 6 engine with a 1 barrel carb, leaded fuel, 3 speed manual with a 4.11 rear end ratio, and 6.00 by 16 bias ply tires!!!!!
Imagine the true meaning of the "KISS" (keep it simple stupid" principle with NO fuel injection controlled by onboard computer, NO catalytic converter, NO AOD transmission problems, and a $ 845 car FOB Destination delivered.
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