I have 2006 nissan exterra. When get to a speed of 40 the rpms go up to 3 and higher loose power and revs loud until I turn the overdrive off the the rpms go back down whats the problem?

Possible shorted transmission harness or transmission shift solenoids. Disconnect the big connector at the transmission under the car and see if the short stops and fuses don't go.

When you check the fluid for discoloration or a burnt smell if it was never flushed you might be burning the clutch plates or your overdrive isnt working if the fluid is good you might have a bad solenoid in your transmission

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..

The cars computer has a rev limiter built in so a no load rev is about 2 grand. but driving the car it should reach 4800 rpms or 5600 rpms if not then install a vacuum gauge on a intake manifold port normal readings should be 16 inches or more at idle then drive car as you accelerate to a higher speed this should drop slightly once at full throttle is will read almost zero if below that then the exhaust has a restriction.

Overdrive is used for better fuel economy on the highway. You have different gear ratios for each gear, for example 1st would be 4.6:1, second 3.2:1 etc. this means for every 4.6 revolutions in, you have 1 revolution out (of you transmission). A overdrive gear will have a ratio of greater to 1:1 (i.e 1:1.6) mean the output is more than the input. This would give you a higher speed at a given RPM, or a lower RPm at a given speed, thus more fuel economy.

Overdrive acts like a final gear. It's easily noticed if it is on or off when cruising at higher speeds, as when operating the switch you will notice the engine revs alternate between higher and lower rev ranges.
When the engine revs drop, then overdrive is on, and when they rise, then it is effectively off.

The overdrive function works just like any other gear in that it reduces the torque from the engine to the wheels, and allows the engine speed to drop. This isn't always beneficial when climbing hills or towing however, or when driving hard, as you may prefer that the gearbox wasn't always changing gear into overdrive, as overdrive reduces the torque from the engine as a means to increase fuel economy and reduce stress. It is for this reason that the overdrive can be disengaged, effectively reducing the number of gears in the gearbox which you have available to you.

So simply put, for most drivers, in most circumstances, they will not need to turn the overdrive off.

In answer to the light on/off, the majority of vehicles have overdrive in the 'OFF' position when the light is on. It's a reminder light (amber) that you likely aren't employing the best fuel saving technique.

You have a fault in the transmission, see a repair shop for further testing.

this is really not a problem, overdrive will only be used if car speed is up to speed for it to engage and that is how its designed. If you pull a trailer or if you are in a hilly area and your car keeps shifting in and out of overdrive you should turn off untill more flater terrain, otherwise it doesn't hurt your vehicle.

No not likely. Is the 3-4 shift rough? What speed are you going when it shifts into 4th? It likely just needs the T.V. cable adjusted.

ram1105: when you shift out of overdrive the transmission actually is in a 1-1 ratio with the input shaft of the transmission. Meaning for every revolution of the input shaft, the driveshaft turns the same number. When you shift to overdrive, the ratio changes allowing the engine speed and input shaft speed to slow down while the number of RPM's increases on the driveshaft shaft. The change is not too significant. Usually about a 500 RPM change is normal. You may have a problem with slippage within the transmission overdrive unit.
It may be advantageous to have the transmission pan dropped and it's contents inspected for debris. Older Nissan Automatics were not known for failures. The 3N71B was bullet proof and the 4N series which was just an overdrive added to it, were very reliable.
My son, best friend and I road race Nissan 300Z's. There are a variety of gear ratios for the R200 differential. The 3:70 is the tallest factory setup on a production Z, which would give you the lowest RPM at a higher speed. Outside of that, the tallest tires you can run on the car will also lower the RPM's.Don't forget there is always a trade-off. When you make a gear and change to taller gears, the car is going to feel more sluggish off the line. It will take longer to attain 0-60 unless you increase HP & Torque. I can help you there if you want to know how to do it.