Ram 5500 w/ 4.88 gears

[LBR]

But the truck in question has a single .71 over drive and the mythical truck has a .63 double overdrive like the .67 ratio of the 6r140 in the 11+ Ford and the .61 OD in the Allison 1000.

That .6x gear ratio in the top OD makes a huge difference in highway RPM.

We are attempting to find a missing 1K RPM (from the stated 1600 to an actual 2500+) RPM and you saying that less than a 1/10th in the tranny final gearing will make that 1K RPM up?

I've been a gearhead for 50 years, so show me the math. Thanx!

 

[NWILSON]

While on the subject of Ram transmissions, here are the numbers for those of us with the 68RFE

5th 6th
.82 .63

I have the 4.10 rear end and the tach needle just touches the low side of 2000 at 70mph

 

[CarterKraft]

We are attempting to find a missing 1K RPM (from the stated 1600 to an actual 2500+) RPM and you saying that less than a 1/10th in the tranny final gearing will make that 1K RPM up?

I've been a gearhead for 50 years, so show me the math. Thanx!

Not really haha. Allot of this stuff I type makes perfect sense to me when I typed it, the next day not so much!

I was just trying to make sure everyone was aware your truck (LBR) doesn't have a double overdrive transmission so it't not really the best test bed for the question. While I agree you can use your truck for the math comparison to get to the final number, you could save the fuel and just done a purely mathematical example from the couch.

Ya'll are all correct though that the OP's myth of a 1600 rpm @ 70+ MPH isn't happening.

 

[SNOKING]

I put the numbers in a gear ratio/speed cal.
4.88 diff
.63 OD
1600 RPM
Tire dia 32.2
speed = 50 MPH

4.88 diff
.63 OD
2405 RPM
Tire dia 32.2
speed = 75 MPH

Chris

 

[LBR]

Not really haha. Allot of this stuff I type makes perfect sense to me when I typed it, the next day not so much!

I was just trying to make sure everyone was aware your truck (LBR) doesn't have a double overdrive transmission so it't not really the best test bed for the question. While I agree you can use your truck for the math comparison to get to the final number, you could save the fuel and just done a purely mathematical example from the couch.

Ya'll are all correct though that the OP's myth of a 1600 rpm @ 70+ MPH isn't happening.

I agree that it wasn't a straight across example as I qualified in my first post, but it seemed like an issue I could help with by pictures to dispell a myth. I personally would rather perform the math calculations for this, but most of this site's members seem to thrive on pictures for guidance rather than "boring math"...lol!

CarterKraft, you are hoot with your reply! This site is such a great resource with such great members helping members...love it!

 

[LBR]

I put the numbers in a gear ratio/speed cal.
4.88 diff
.63 OD
1600 RPM
Tire dia 32.2
speed = 50 MPH

4.88 diff
.63 OD
2405 RPM
Tire dia 32.2
speed = 75 MPH

Chris

Thanx Chris...that is awesome and my preferred way is by calculations...because I was going that 70 in a 55 for that picture... You can see that the "door ajar" light was off at 43 MPH, but was on at 70....that was the DW getting ready to bail out unless I slowed down and put my phone down...hahaha!

The calculations would be pretty close to my actual as I believe my tires are 31.6 per the mfg. My shorter tire than the 32.2 calculated would affect the MPH and RPM to close those gaps some too.

 

[dave10a]

Was told recently about a fella who bought said truck (used w/ RV flatbed) and the Asion tranny and claims that at hiway speed (70-75), RPM's were below 1600. I didn't think anything above a 3.31 ratio would be that low of RPM's. Is this possible to do w/ the Asion and 4.88? Or are there extra gears within the Asion trans. Claims it's a 6 speed. Plus, the Asion is a Allison in disguise built by Allison, but can't call it that due to Allison being pared up w/ D-max/GM. IDK- just from what was told.

I think that when it comes to tow vehicles, many folks accept meta-physics over real physics.