adicus8

I did a search on this and could not find what i was looking for.

I know it stands for over drive, but i dont know exactly what that is. I have pushed the button before and drove around and noticed something different with the rpms. So, what is overdrive, and in what situations would I turn it off.

I am going to be on a 4 hour freeway drive pretty soon, and last time i did this drive my jeep struggled to stay at 70 mph. I would slowly loose speed, then have to give it more gas( higher rpms) to get back up to 70. I am sure this is bad for mpg, and it is also frustrating not being able to hold a constant speed. Would over drive off help? How does O/D off effect mpgs?

Thanks

dlud51

Generally speaking, overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h [40-45 mph or more] depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears. For an automatic transmission, it is almost always best to select overdrive and allow the transmission to control engagement of the overdrive. (It may be necessary to switch it off if the vehicle is being operated in a mountainous area or when towing a trailer.)

Hexavalent

I dont really know that much about it but from my understanding when you turn the overdrive off something slightly modifies the gear ratios to keep you at higher rpm's in each gear. I dont have an automatic so I dont really have much experience with it but im pretty sure its commonly used when towing. So if youre having trouble keeping highway speeds I think the overdrive being off would definitely help.

l00py1211

Overdrive is just a fancy word for high gear. So for you that would be 4th since you have a 4 speed automatic transmission. When you activate "O/D OFF" your transmission downshifts to 3rd gear. Therefore running at higher rpm, which will make is MUCH easier to hold 70mph, but there will be a noticeable decrease in MPG...

nthinuf

OD On lets the transmission use 4th gear.
OD Off locks out 4th, so all you have is 1st - 3rd gears.

To put some numbers to it -
I'll assume you have 3.73's and that the actual mounted height of those 33's is 32".
Auto, 3.73, 32" = 1892 rpm @ 70mph
- Divide by .69 to get OD Off -
1892 / .69 = 2742 rpm @ 70mph

On the freeway, if you are downshifting a lot or can't hold speed, turn it off. Around town, there really isn't any need to be in 4th gear so turn it off.

Noxian

http://auto.howstuffworks.com/automa...nsmission8.htm

adicus8

awesome, thanks everyone. I will drive with it off this time and hopefully my mpgs wont be too horrific. I might do a comparison, on the way to my destination i will drive with o/d off and on the way back i will drive with it on. I will put my mpg on here afterward.

roadking54

...you realize what this conversation eventually leads to don't you....re-gear problem solved!