85_Ranger4x4
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The thing about having to downshift is very true. While a truck should getter better mileage near it's torque peak, it's not making peak torque unless you have it at that particular rpm with the pedal to the floor and it isn't accelerating any more. Any lower torque setting (part throttle cruise) and I believe the lower rpms mean less fristion and the other factors mean less. This theory is born out by poring through the EPA's website and looking at the year before and the year after overdrive showed up. That's 1981-82 depending on the vehicle. Overdrive adds maybe 15%--so anytime you pop out, you are losing that benefit. If you have a wiffle engine that has run run wide open to pull a hill, then it isn't as important.
Having a stick would help alot too with downshifting, it seems with an auto 80% of the time it downshifts you are almost over the hill anyway, 100 or 200 fewer RPM's wouldn't hurt much. Sometimes I will set the cruise a little lower than normal and go up hills with the throttle, kind of odd that it seems more reluctant to downshift then.