Northstar Engines and System Technical Discussion Discussion, overdrive or drive in Cadillac Engine Discussion; Just a question for everyone. I once heard that you should drive your car in drive if you are under ...
- 01-12-04 02:59 PM #1Cadillac Owners Member
- Join Date
- Jul 2003
- Boise, ID
overdrive or drive
Just a question for everyone. I once heard that you should drive your car in drive if you are under 45mph and overdrive if you are driving over 45. Any thoughts on this?
- 01-12-04 03:07 PM #2
Re: overdrive or driveOriginally Posted by 51magnum
- 01-12-04 05:20 PM #3
Re: overdrive or drive
This is a slight misconception.... Allow me to clarify...
First off, JJhomer, your eldo should have OD... IT may not necessarily say so, but Im very sure yours has it... Over drive is simply stated as the engagement of the torque converter... I dare say most auto trannys have this..... You may not have an option for D or OVER-D, but if you only have one D, then you have overdrive on all the time... Some cars have a thing where you can turn it off.....
51- You are correct, it is best to drive with OD off under 40-45... If you dont have this option, then you put it in 3.... Gas mileage may be slightly affected, but the less shifts the tranny has to make the better.... ANy little thing you can do to keep that going strong is good!!!
- 01-12-04 06:26 PM #4
Re: overdrive or drive
The GM computers are pretty good about this. The older Ford AODs were horrible, if you were doing 35, it couldn't make up it's mind wether it wanted to go into OD or not. OD is a little more than the torque converter locking up, its a whole other gear ratio that is below 1:1, like 0.85:1 or something like that. Some cars do not have an automatic overdrive system, and they always have a lesser ratio in all of the gears when OD is on.
- 01-12-04 08:03 PM #5
Re: overdrive or drive
A transmission is in overdrive when the revolution of the trans output shaft is more than the revolutions of the engine. For example, when the trans is in first gear, the ratio may be 2.57 to 1. That means the engine has to turn 2.57 times for the output shaft of the trans to turn one time. As you go up through the gears the ratios are closer so the engine has to turn less to equal the same amount of rotations of the output shaft. The next gear may be something like 1.51 to one. Third gear is usually one to one, which means the rotations of the engine match the rotation of the output shaft. When you hit fourth gear, that's usually the overdrive gear. The ratio may be .067 to one, which means the engine is turning less than one revolution while the out put shaft turns once. That's where you save gas because the engine is just loafing along, so it uses less.
Just because the torque converter locks up doesn't necessarily mean you have an overdrive trans. It's the gear ratio that makes the difference. A locking torque converter means it's eliminating slip loss and allowing the power to go directly to the trans at higher speeds.
In a nutshell, if you have four forward speeds or more in your Caddy, you have an overdrive trans. If you have three speeds forward , it's not an overdrive trans. Caddys have had four speeds from the 80's on, so you have an overdrive trans.
If you're driving on a hilly, twisty road, Cadillac recommends keeping the trans in 3rd gear to keep the shifting down and the torque up. Around town, just leave it in drive.
- 01-12-04 10:00 PM #6Cadillac Owners Fanatic
- Automobile(s): 2004 Cadillac Escalade
- Join Date
- Jan 2003
- South Florida
Re: overdrive or driveOriginally Posted by elwesso
"Overdrive" just means that the tallest gears in the transmission are less than 1:1 ratio. The normal top gear in a transmission is 1:1 ratio. Over drive transmissions have an extra top gear that is overdrive at .75:1 for example.
The overdrive name refers to the fact that the output shaft of the transmission is turning faster than the input shaft when in these overdrive gears.
The torque converter clutch is a completely different issue where a clutch plate is activated to lock the turbine of the torque coverter to the case of the torque converter basically effecting a solid lockup of the converter - eliminating any of the normal hydraulic activity inside the torque converter - and eliminating any slip that hurts efficiency.
Most of the FWD Cadillacs have a special lockup torque converter which includes a viscous coupling - hence the name VCC for viscous converter clutch. It locks the unit the same way. But, the viscous coupling allows some slip to eliminate the chuggle often associated with lockup converters.
As far as driving in overdrive or not, the transmission is calibrated to operate with the lever in the highest gear ie. overdrive. Just because the lever is in overdrive doesn't mean that it'll go into overdrive. The transmission shift calibration is setup to select overdrive at the appropriate time depending on speed and load. There's really no need to try and second guess it. Coincidentally, the 3-4 shift (or the shift into overdrive with the 4T60E and 4T80E transmissions in the Cadillacs) is set up to occur about 42mph at minimum throttle. So, the factory sets the unit up to shift into overdrive at a minimum speed of 42mph automatically.
The only real exception to this is if you are towing a trailer and the transmission is shifting constantly on hills or even on the flat due to the load. Usually it's a pretty safe bet with a trailer of any size or mass to put the lever in 3 (not overdrive) and leave it there to eliminate overdrive operation at any speed. The engine will operate at a higher RPM but at a lower load and there will be less load on the transmission.