518 overdrive and 3:07 rear

[rdodd]

Does anybody run the 518 overdrive auto trans. with 3:07 rear? I sometimes pull just a small camper and was wondering how these gears would compare to my current 3:55 rear as far as pulling or starting out from a stop. I think i would like these gears for cruising empty and also i could drop it out of overdrive when pulling. With 3:55 rear tach reads about 2000 rpm at 70. What would tach read at 70 with 3:07? Also, what would tach read at 70 with od switched off with the 3:07?

 

[Philip]

I have the 3:07's with a 518 in front of them. I do not have a tach so I can't give you RPM's at whatever speed.



I have towed a 8k load with mine. That was trailer and the car on it. With a stock loose converter I ran out of motor at 70 going threw the MO state. So now I limit myself to 8k to 10k with the gearing I have.



For most of my drivng. The 3:07's are good. Empy fuel milage is good. The engine isn't working hard till around 80 MPH +. I top out against the governors at 89 MPH in third. In OD it is above the speedo markings.

 

[bgilbert]

RD, sorry for teeing on your post but while we're talking bout the 3. 07 gears how bout max speed/top end with 3. 07's and getrag 5 speed? I have the 3. 07's with the 727 3 speed auto and top end sucks, only like 70mph+-. Bill

 

[Forrest Nearing]

divide RPM by 3. 55 and then multiply by 3. 07

 

[Doug Czaja]

bgilbert,



You have mail..... or you should if I didn't mess it up .





Doug

 

[bgilbert]

Doug, no mail yet. email -- email address removed -- Bill

Forrest, looks like you drop about 350-450 rpm each time you drop from 4. 10-3. 54-3. 07? Is this bout right? Say 2600 rpm divide by 4. 10 x 3. 54 or 3. 07 = 355 rpm? Or 4. 10-3. 07=drop of 653 rpm? How much speed do you gain with say 350 rpm gain? Bill

 

[Forrest Nearing]

well, it's not a linear jump... at higher speeds, it'll be a bigger jump...



here's a tool I use often when calculating optimal tire height and gear ratio for our drag cars... go to - www.randysringandpinion.com - click on "calculators" on the left, and you'll find a few handy ones... there's one for calculating engine RPM based on speed, tire height, and RPM. very handy!



Forrest

 

[steveinaz]

518+3.07's

You will have to modify the valve body and governer in the trans to get enough oil to the od unit . The line pressure in a 518 is rpm sensative via the governer. These modifications should be done with 3. 54 and 4. 10 gearing.

 

[cerberusiam]

In stock form the top speed witha getrag and 3. 55's was 96 mph, with a 727 89 mph, with a A518 top speed was 92 mph. The 3. 07's will raise top speed some but it won't be linear due to the engine being at its upper limits of fueling. Bgilbert, 70+- sounds way short of what the truck I drove would do when it was new.



As steveinaz said, oiling the OD is going to be a big problem with that high a gear ratio. The 518 OD is way short of cooling to start with and it will definitely generate more heat the higher the gearing goes. The OD on the 518 is at . 69 already so you would really get some long legs. That unfortunately just accentutes the weaknesses in the early trans.



If you are running 2000 rpms at 70 mph that should be about the optimum for engine efficiency with hammering everything with TQ peaks and the engine vibrations. I have tore up the 727's running 3. 23's and hopped up gassers that would not approach the TQ of a stock Cummins. I would have some serious concerns hauling anything with that high a ratio. Even running empty it is going to stress things more just moving the weight of the vehicle.

 

[rdodd]

I would think 3:23 gears would be nice in our ctd,s but i have been told that they aren,t available for the dana 70. I really should just keep the 3:55 gears i have now since i do some towing. A friend of mine has 3:07 rears in his 92 5-speed and runs in forth when pulling heavy. He loves the gears. He said it works out to be about the same rpm,s in 4th as running 4:10 gears in 5th.

 

[914x4ctd]

I have 4. 10 gears in my 91 w250 with a 5 spd manual. For some reason my speedo was off when I had the stock tires. It read faster than I was really going. I then installed 33" tires and my speedo is right on the money. I checked with GPS and friends new truck and both numbers were right on. Has anybody else had there speedo read fast with stock tires and 4. 10's? top speed is 83-85 mph floored. But can drive 65-70 ok. 70mph=2100 rpm. My tach seems to vary by about 1000 rpm at times is that normal? Does anybody else have a Banks tach? I have the only one that I have seen.

 

[Forrest Nearing]

sounds like your truck may have had a speedo gear for a 3. 54 rear in it.



Forrest

 

[914x4ctd]

that is exactly what I thought.