I may be lazy looker but this seemed like the best forum for this question. i just hope there are car people here. i have a 1993 GMC sierra V6 truck (dark blue, in case you were wondering). On the "shifter" i have the option to drive in "drive" or "overdrive." Now i don't really understand the difference too well. All the sites i have read to a rather poor job of explaining it: usually they use words that i don't understand or they are too make/model-specific for it to apply to me. on my way to school every morning, i drive 4 minutes at about 60 mph and roughly 4 minutes at 45-55. The road is relatively straight with little to no traffic or stoplights until the very last minute or so. From what I have understood, overdrive actually REDUCES the work done by the engine (in terms of RPMs) and INCREASES the gas mileage. Could someone outline what is going on AND tell me when (if at all) i should use OD (including in other situations). gas costs $2.65 and my truck is 12 years old...I need to become as efficient as possible.