Interesting result while resetting the pinion factor on my 05 3500 SRW today. The service writer (Tim) brought the DRB III out to the truck, hooked in and we paged through.

Misc.
Tire sizes (other)
rear axles (other)
Pinion factor. reset from 657 to 640.

I expected some small error as the Cooper 265R70-17 Discoverer ST's are still on the truck because of the 12 miles of icy mountain roads I had to transit on the way home. We were setting the pinion factor in anticipation of mounting the new 33x12.50-17 Discoverers in a couple of weeks.

The overstating error on the speedo is far more significant than expected. Instead of 5mph too high, it jumped to 15mph too high.

So the question is: did we err by punching in "other" in rear axle selection?

The choices are:

10.5 blah blah
11.5 blah blah
other

pinion factor was the only selection under other.

 

This is just my experience based on 2nd gen DRB play-time! After you select the correct axle, do you get options for different tire sizes (at least for the factory-offered tire sizes)? On my 2001, the menu for tire sizes included the two factory options as well as an option to input a number for rotations-per-mile. My buddy the super-mechanic said the axle options were for different tone rings on different axles...

 

I recall only about two tire sizes on the first menu item under misc.

and then "other"

Which brought up two rear ends and another "other"

We went into the "other" menu and came up with pinion factor as the only entry.

Going from 657 (existing setting) to 640 should not have made such a big change in indicated mph. Right now I'm reading 75mph at 1700rpm with 265R70-17 (I wish!!!!)

Normally this would be about 60mph. I expected it to jump to 62-63mph which would be about right with the new tires.

 

I believe your rear axle selection should have been "11.5" as we have American Axles with the 11.5" ring gear. They put the 10.5" ring gear units on the Hemi rams.

 

I was worried also, that we chose wrong by selection "other" at the axle menu. Today I mounted the 33x12.50 Cooper ST's, fired up the GPS and went for a drive.

58.9 on the GPS when the speedo said 70.

Okay, so a 3% correction on revs per mile nets a 20% change in indicated mph.

Back to the dealership. This time the tech came out, scrolled through the menus, selected 11.5 axle and went to pinion factor. 640, just as we had set it. He said doesn't matter how you get there, pinion factor is the same.

We checked my 640 number with the tire manufacturer and it was "correct".

What now? the tech asked.

17 revs per mile change was a 20% change and we needed 5%.

I said, make it 352. He said, "You're just going to pick a number?"

Why not?

Five miles down Hwy 35 at 70mph indicated came out to 70.1mph on the GPS.

Go figure.

Moral of the story is: don't think revs-per-mile numbers are going to make it. Use a GPS and plan on at least two adjustments on the DRB.

However, using the revs-per-mile sure does give some fantastic mpg numbers on the overhead console.