O/D ratio in automatics

[Steve M]

Just sitting here having a cup of coffee wondering why automatics have a . 69 O/D ratio and standards have a . 78 O/D ratio or there about. The . 69 ratio is nice for cruising empty but sure is tall when towing. I can buy different ratio gears for my Harley both internally and final drive. I'm surprised one of our transmission specialists haven't come out with an O/D gearset for autos that closer matches that of the standard transmission. Just wonderin'...

 

[Gary - K7GLD]

By the time you bought and installed the different OD gears, it might not be much more cost effective than installing a US Gear or Gear Vendors OD setup...

 

[LSMITH]

Originally posted by Gary - KJ6Q

By the time you bought and installed the different OD gears, it might not be much more cost effective than installing a US Gear or Gear Vendors OD setup...

Nah, it really is not that big a deal. Not something for most shadetree types, but if the gears were available LOTS of us would have done it.

 

[ronco]

Just my guess, but the O/D ratio in automatics is done with planetary gears and therefore the ratio might be the only ratio that works out. There may be size constraints within the trans to prevent other ratios. Personally, the manual trans owners might be jealous of the autos superior O/D ratio



Ronco









P. S. If you bought into that last line, I have property for sale

 

[Steve M]

I don't think the ratio thing is a problem. The aftermarket O/Ds use planetary gears. My thoughts are that they still haven't made the change from the days before locking torque converters. O/D was tall to make up for the fluid coupling loss in the automatics. I had a 5spd in my second Dodge Cummins. It was great for the most part but I got tired of spring wrapup while backing our 5th wheel into the yard and elsewhere. It was a little grabby when it was hot after a long pull too. But it sure sounded killer! So I'm back with an auto and the 3. 54 ratio lets me tow 55-65mph in 3rd when the road gets hilly. The O/D unit attatches to the rear of the 727 3spd transmission much like an auxilliary O/D making it a 47RE. I'd just like to see a closer ratio O/D unit offered to help close the gap between 3rd and O/D a little. No telling what kind of market there would be for such an offering. I would buy... mmm... one!

 

[DIESELMAN]

There is no seperate overdrive gears in the transmission, and there is no real size constraint to getting other gears, its just what works out.



A 48RE has a typical compound planetary gearset. I think the Ring gear may be about 74 tooth, the sun gears are 30 in the first and 32 teeth in the second. Somebody may have the exact count.



The two planetary gearsets work together to get the needed ratios. This gives a ratio of 2. 466:1 in first, 1. 44:1 in second, 1:1 in third and an overdrive of . 698:1.



This is pretty close to the spec ratios. of 2. 45 / 1. 45 / 1 / . 69



The point being is that you can't change one ratio without affecting the other two. The OD ratio as well ast the others are a compromise in gearing. A good balanced transmission that is strong enough. At least this part (planetary set) doesn't seem to fail very often.



If someone could machine something like 80/26/28 planetary gear set they would have very close 2nd through 5th of an NV4500 or 2,4,5,6 in an NV5600. I don't know how well that would work or even if it could be done or be strong enough. The Ring would be too thin with teeth too small, and the suns would be too small. This would probably be too fragile and not hold the torque.

 

[Steve M]

I beg to differ with you on the O/D unit. My shop manual states that removal of the overdrive unit is the first step in the process of rebuilding the transmission. It bolts to the rear of the transmission where the long tail shaft used to bolt to. In a different chapter is the rebuild process for the overdrive unit. It is a separate unit with oil lines and a linkage run to it inside the two units. It appears that it would be easy to swap out if such an animal were available.

I could have used a closer ratio O/D today coming back from the inlaws' house towing the 5'er. It was either 1750rpm in O/D or 2500rpm in 3rd. The headwind and hills made O/D too tall. So we listened to it howl like a banshee for four hours in third. 2100rpm would have been nice with third available for the steep hills in 60mph speed limit areas.

 

[DIESELMAN]

Well then, I beg to differ with myself, if thats the case then the 46/47/48(42?) transmission's are just like the older torqueflite 727's, a three speed with a seperate planetary overdrive.



BTW, I'm not a mechanic, but I do know how to fix anything I've broken at least once Even when it wasn't broken to start with.

 

[Steve M]

Yes I believe that's pretty much what they are. A greatly improved 727 with an O/D in the tailshaft. The 727 was a tough transmission. I have high hopes for the 48RE.