I bought a 96 Ram 3500 Diesel dually. Soon on the road I discovered that I have great speed - I can easily get to 90 mph - as long as the OD is on. I know that once I the OD is OFF it will go up on RPMs and slow down. It's normal. My problem if that without any load, with OD OFF on the interstate I cannot get it to go beyond 60 mph at 2500 RPMs. If I floor it, the RPMs keep going up but the speed stays at around 60 mph. The transmission is fine, it does not slip and the gears shift when they are supposed to. It has a 4.10 gear ratio. My question is: If I change it to a 3.54, how much more speed am I going to gain with OD OFF.
Is there a little formula? The ratio is about 16% smaller; does that mean that at let's say 2500 RPMs the truck is going to go 16% faster that before at those RPMs.
Any help would be appreciated.

 

I'm a little confused as to why you don't want to use overdrive. With the 4.10 gears using overdive while towing should not be an issue, if that is your issue.

 

I've got the 3.54 gears in my 01 and I run about 1500rpm at 55mph. in 4th gear. I couldn't tell ya what it is in 3rd. But yes, you will be running 16% lower rpm with the 3.54s at any given speed.

 

Checked the RPM at 60 today with the od off- 2150ish. So that would give you right at the 16% you mentioned. Why you can not accelerate any more is beyond me. I can run it up 3000 without it hesitating.

 

Bass Hunter, that is exactly what I was doing, once I got to speed I would switch to OD and coast at 70 mph, but I burnt my transmission and the mechanic told me that it was because of what I was doing - that I should never switch to OD while pulling.

 

Scott - It is beyond me too...I still cannot conceive why by the time I hit 60 @ almost 2400 RPM I can take it to 2800 but the speed does not increase. We test drove it witrh the mechanic that replaced my transmission and the only though he had was about the gear ratio.

 

I pull a 14,000 trailer on a regular basis in tow haul mode wich still allows overdive at cruising speed and I have never had an issue. This is with 3.75 gears in the rear. You do understand that by switching to 3.54 gears you will be working the trans and motor harder while trying to accellerate.

 

Bass Hunter - I totally understand and I would like to keep the 4.10 if I can. Another part that my transmission guy had to replace was the throttle positioning switch because it was not sending a signal to the transmission. Would you think that the switch could have been the real problem?

 

I would think that would explain the lack of acceleration past 2500 rpm. Have you considered doing some tranny upgrades instead of the gear swap? Like an aftermarket torque converter and upgraded valve body as well as another auxillary cooler with a fan?

 

As I mentioned, I just replaced the transmission and installed a heavy duty torque converter. I though - out of not knowing much about the matter - that my new xmission was going to resolve the speed problem. Obviously it didn't. I guess it's a few $$$ late to replace the TC. But I have not thought about an upgraded valve body and aux cooling. Any suggestions on that?