I usually use my overdrive after i get higher than 60km...i was told not to, but it backfires when I don't what will happen if I continue to use it?

Taurus are notorious for tranny problems,get rid of it

Over drive is a gear that is used for constant higher speeds. Usually will kick in automatically on almost all vehicles. Over drive is designed to save you gas at higher speeds.
I would suggest not turning it off unless you are towing or climbing a steep mountain (something that would strain an "Over drive gear).

lots of mazda vehicles have a small button on the shifter, usually on the side or somewhere not very visible. Generally overdrive is actually ON and pressing the button will turn on a light that indicates Overdrive is OFF. since overdrive is kind of a passing gear - if you are going uphill or towing something and do not want the vehicle to shift to a higher, larger gear, you would turn overdrive OFF .

usually on the end of the cruise control button(lever)..but look at transmission fluid levels add if low..overdrive is the first gear you loose when fluid levels drop..

It is automatic, you don't have to do anything.

as a rule, no. many overdrives are automatic and don't engage until you reach 40- 45 mph and they do this by themselves. yes, this does affect gas mileage because the purpose of overdrive is to shift the transmission into a higher gear producing less strain on motor and ****** thereby increasing fuel mileage. conversely, if ****** is in high gear at lower speeds this increases strain (motor feels like it's lugging) reducing mileage. sounds as though you have manually selective overdrive and you will have to monitor mileage both with button pushed and without. then try pushing the button only when you reach 40-45 but you'll have to be continually conscious of the speeds at which you're driving.

i was told that some rangers have a govner or sensor that won't allow you to go over 3,ooo rmp at a idle,it will make it run rough and backfire,it' to keep you from blowing up your motor

What is happening is the tranny is going. They can try to replace the selenoid, but really, you need a rebuild. Whomever charged you to remove the light, probably messed things up more. Volvo's typically lose overdrive before losing all the other gears. It is time for a rebuild. Go to a reputable tranny shop who knows volvos.

it might be the overdrive cuz my dads truck does that and he presses the overdrive button and it stops then when he is at hwy speed it starts shakeing again then he turns the overdrive back on then it stops

sounds like a choke sticking or carburetor needing attention..(adjusting or rebuilding) hard to say more without hearing it my own self.....hit me up if i can help more [email protected]