Over Drive

[retrokid_23]

When is the best time to use over drive when driving my xterra. Do you lose more gas when you use it?

[ChuckH]

The best time to use it is pretty much all the time. Using OD saves gas and makes the drive more pleasant. The only times you really want it off are when towing or when you want to use engine braking coming down a hill. Not sure what other reasons there would be to turn it off, but I'm sure one of the slush box guys can chime in.

[OffroadX]

Yup, in general, leave it on for routine driving.
Turn it off when towing, though if you're truly loafing along at steady speed on level terrain at the usual RPM for a given speed, you're OK.
Turn it off when climbing long grades that see the engine RPM jump like a downshift but it's not really a downshift (explained below).
I'd suggest leaving it off in situations like traffic or winding roads where your speed varies and doesn't get above 50 for any length of time. Off when passing/merging too if you really want to be thorough.

What you need to familiarize yourself with is the difference between overdrive (just 4th gear really) with the torque converter in lockup, overdrive with the TC out of lockup (often mistaken for a downshift), and 3rd gear (torque converter will almost never unlock in 3rd). Best way is to pay attention to your engine RPM at a given speed, say just under 2500 RPM @ 65 MPH when loafing. If you give it a good shot of gas or start up a grade @ 65, the RPMs will jump right up to around 3000 or a bit more without a corresponding increase in speed right away. This is the TC unlocking, not a downshift. Immediately turn the overdrive off, if there is another "shift" and anoter jump in RPM, then you know this was the actual kickdown to 3rd and the previous one wasn't.
When the TC is unlocked, the TC input spins much faster than the output which rapidly creates a lot of heat in the fluid if it continues for more than a few seconds. Make it a point to recognize the TC unlock and even to get a feel for when it is about to happen, and turn the overdrive off if it appears the unlocked condition will be more than brief.

In 3rd gear, the TC will almost never unlock above 45-50 MPH. About the only time it will is if you turn the overdrive back on while still heavy on the gas, the RPMs will jump REAL high when it does. This is not something you should allow to happen, and it won't shift up to 4th until you really get out of the throttle. Meanwhile it will heat up the fluid in no time.

Brent