and do I need it to be on or off? I borrowed an Ford Expedition from my BIL, and I can’t get ahold of him to ask and his owner’s manual isn’t with the truck.
We’re taking a road trip and I don’t want to do damage to his transmission, so should it be on or off?
thanks
H8_2_W8,
The short answer is, yes you should use overdrive on your road trip. Just flick the switch, press the button, or shift the lever, whatever it takes to put it in OD. Leave it there for the duration of your trip and forget about it. You’ll burn less gas this way.
The longer answer (the one I was gearing (Hah!) up to give when I saw your title Q) is that Overdrive is just a higher gearing in your transmission. You no doubt understand gearing in a car, at least to the point that you know to start in 1st and shift up as you go faster. Well, what’s happening in your transmission is that each time you upshift, you’re decreasing the gear ratio in your transmission (or the trans is doing it for you if it’s an automatic) Let’s say 1st is 10:1 (I’m not sure if that’s high or low at all). When you shift to second it goes to maybe 6:1, third to 3:1 and fourth to 1:1. What that means is for every 10 revolutions of the input shaft of your transmission, your output shaft spins 1 time in first. In second, its 6 revs input to 1 output, and so on. If you run your engine at a constant RPM, then the output shaft goes faster and faster each time you upshift.
Now, in 4th gear at a 1:1 ratio, the output is going as fast as the input, that’s direct drive. Let’s say that ain’t fast enough for you and so we’ll put in a fifth gear where the output shaft will spin faster than the input shaft, maybe 0.5:1, so every rev of the input gives two revs of output. This is known as Overdriving the transmission, and that’s exactly what your Expedition will be doing on this roadtrip.
Why the button, you say? I think (but don’t know) that an electronically controlled automatic transmission will use a different shifting pattern with the OD engaged than without. Without the OD, you’ll be faster off the line, but with the OD engaged, the shift points are at a lower RPM. That’s what I observed in a Toyota Camry I used to drive.
Hope this has been helpful and not too long-winded.
Great explanation, except your numbers are way off 
More realistic gear ratios:
1st-- 3:1
2nd-- 2.5:1
3rd-- 1.75:1
4th-- 1:1
OD-- 0.6:1
Basically, overdrive is a higher-than-high gear, for cruising.
in relation to what the other two said on automatic transmission the option to turn on and off the overdrive is so that when you are pulling something then you want the overdrive off so that you don’t stress the gears in the higher settings. But if not pulling somthing the you should have it on. It’s just like a standard. Would you want to be in 5th gear when pulling something or in 4th?
There’s another reason to turn off the o.d. If you’re in the mountains, turn it off. An overdrive works on a planetary gear/overrunning clutch device, so if you go downhill with your foot off the gas, the o.d. will freewheel and the engine will offer no braking to slow you down. All braking will be done by the brakes. This is bad because they’ll overheat after a while. The brake fluid will boil, you’ll have no brakes, and you’ll become a blot on the side of the mountain.