Trans Help
Postmaster
Joined: Apr 2002
Posts: 3,613
Likes: 6
From: Coal country
UL37: The meaning of the term "overdrive gear" has been blurred, so your confusion is understandable.
"Overdrive" means the driving component is turning slower than the driven on. That's all.
In the '30, a planetary gear overdrive controlled electrically was developed. The concept didn't change very much all the way to the '70s. Many people still think that an OD gear is something special, an addition, and do not realize it can be just another, higher gear, higher than 1:1.
Steeply rising gas prices in the middle '70s (remember the Arab embargo??) revitalized interest in fuel economy. The concept was introduced that the 4th gear should be higher than 1:1, and thus a new means of OD was born.
Computer and electrical developments for the automatic transmissions were ongoing. Early problems were functionality and longevity with lock-up torque converters, solenoids, burnt up transmissions because shift cables were only slightly out of adjustment, and the like.
Fortunately, much of that has matured, and they work very well and last long now.
Hope this clarifies your questions.
"Overdrive" means the driving component is turning slower than the driven on. That's all.
In the '30, a planetary gear overdrive controlled electrically was developed. The concept didn't change very much all the way to the '70s. Many people still think that an OD gear is something special, an addition, and do not realize it can be just another, higher gear, higher than 1:1.
Steeply rising gas prices in the middle '70s (remember the Arab embargo??) revitalized interest in fuel economy. The concept was introduced that the 4th gear should be higher than 1:1, and thus a new means of OD was born.
Computer and electrical developments for the automatic transmissions were ongoing. Early problems were functionality and longevity with lock-up torque converters, solenoids, burnt up transmissions because shift cables were only slightly out of adjustment, and the like.
Fortunately, much of that has matured, and they work very well and last long now.
Hope this clarifies your questions.
Last edited by banjopicker66; Jan 26, 2007 at 05:41 AM.
Postmaster
Joined: Apr 2002
Posts: 3,613
Likes: 6
From: Coal country
By the way, transmissions with an overdrive gear made after '72 do not have a planetary overdrive gear system. They are a normal manual transmission, and can be treated like any other manual transmission.
They do not roll in gear, and do not "have" to be put into reverse gear when parked.
They differ conceptually only in the final gear ratio.
They do not roll in gear, and do not "have" to be put into reverse gear when parked.
They differ conceptually only in the final gear ratio.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Originally Posted by ul37
now that i'm on the same page here(replied to the old post didn't look at the date).i understand th princpil of od trans and that they can be electric. i am running an srod trans(tremec t170fs) and 4th gear is .78 or .79 to 1 and 3rd is 1 to 1 and when i throw 4th rpms drop about 1000 thats not an od trans (got the ratios from a trans web site) ? i do not mean tobe disrespectful but i do like learning new stuff.also i was born the same year as my truck 66 (thanks for setting me straight on that ND) so i never even saw an orig electric od trans only heard of them.
First off you have a 3spd with OD tranny. That is why the ratio is 1 to 1 in 3rd or high gear.
When you shift into OD or the 4th gear the output shaft of the tranny is turning faster than the engine, therefor the engine slows down. The vehicle is moving along at speed so it is easier to maintain the speed.
John
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