when to use overdrive.

okay, so im not exactly proud of this question but when should i turn overdrive off and on? i mean ive never had a car with it before, i suppose i should drive with it on, right? why else would the light only come on if its off? anyways, excuse my stupidity and help me out.

Overdrive is essentially an extra gear. I keep it off for city/hills driving, turn it on for the highway.

Having it off means that the tranny won't shift to overdrive, keeping the tranny in the lower gear and the engine RPMs up (which keep you in the power band, giving you quicker acceleration)

ahh, i see. any damage if you use it on in the city? and all the time for that matter?

Its designed to save gas, so having it off will generally use more gas. However, it might be different in the city depending on your driving style.

Tell us the make model and year of car!

Some cars such as GM models up to mid- 1987 operated best if you lock out the cruise in the city.
For most cars it does not matter if you leave it in OD all the time. No damage will occur.
Wolf is correct on the useage. Locking out OD makes for smoother and more controlled driving over hills and especially if you are pulling a trailer.