i think overdrive is basically meant for cruising speeds like the hwy to lower your rpm and increase mpg. probably should be turned off for city driving to give u extra rpm to get around faster. if i use my moms explorer its on 99% of the time. i'm never in that much of a rush that turning off the overdrive will get me there quicker. on rare occasions i'll turn it off, even on the hwy to get extra oomph on onramps then i just turn it on again and cruise. downhill engine braking, not sure if it should be on or off
