Hi. My '88 Ram D100 w/904 tranny has incorrect speed reading. When I'm doing 65, actual speed (using GPS) is 58. Crawled under truck to pull out speedo pinion gear to see what number I have. The pinion gear housing (the part the speedo cable screws into) has 21 to 25 markings on it. No other number combo's on it. I know the identical part on the 727 tranny has 26 to 31, 32 to......to whatever and so on. The gear that came out of the '88 was a number 25 gear. I compared it to a speedo gear that came out of my 727 tranny. The '88 904 gear is the same length but the gear itself is SMALLER in diameter than the 727 gear.

From doing a visual comparison, I question if the 904 pinion gears and the 727 pinion gears are interchangable. Instead of looking for a #26 or #27 pinion to fit a 727, I need to find one to fit an '88 904??? Is this correct?

Next question: Since the '88 904 pinion housing only has the markings of 21 to 25 on it; if I put a higher number gear into the tranny, do I need to rotate the pinion gear housing or....or leave it in original position as thou it still had the #25 gear in it???

the housing is offset so it rotates toward the output shaft to move a small gear toward the output shaft and rotates away from the output shaft for a bigger gear.

you will have to fit it by trial and error maybe... not sure about your #s and alinemnt.

I pulled a 27 tooth gear from my 904 and poped in a 38 tooth gear from my 727 when i swap rear axle gears.

the speedo pinions should swap unless it has the newer mid 80 style with a plastic housing and a hairpin clip holding a different style cable & gear on it. ( 88 gran fury 904 for example)

I have some spare housings and pinions and they all swap out from 904 to 727 depending on the speedo cable hook up and rear axle gear.

to adjust one,a small gear speeds it up and a large gear slows it down on the cluster.

you need a bigger gear to slow the cluster down to 58 mph from 65 mph

i would look for a 27-28 tooth gear.(IIRC I have some of the 27-28 tooth gears

you may even need to change the speedo housing to use the bigger gear..can you post a pick of the speedo housing and gear?

I have a 27,29,32,35,38,40

my 88 d-100 had a housing and gear like in the pics.but I dont recall what speedo gear #,it would have been small due to the 2.9? something gear ratio.

What Scratchn said, need a bigger gear (more teeth) which'll turn slower to get the speedometer from 65 to 58 and get another housing with the 3 sets of #'s like what's in the pic so you can rotate it correctly on installation which is a must. Never seen one that did not have all the #'s on the outside. I laid a piece of stock across the top of a rear tire & got my actual diameter then used the chart in the archives along with my gear ratio to get the correct tooth count I needed which made my odo come in correct checked at the interstate mile markers (went 5 miles) and also my speedo came in correct measured at a roadside digital MPH sign. With the odo corrected with the right tooth speedo gear the magnetic gismo in the speedo head can still read incorrect which means the speedo head needs to be taken to a speedo specialty shop to be recalibrated but as you know the speedo is triggered from the odo but likely the correct tooth gear will make both read correct

Here is the pic from underneath the '88 904 tranny.

Here is the speedo housing from same tranny.

Here's a pic of the 25 tooth gear that I removed from 904.

One more shot of it.

If I remember correctly, at least on my 87 Diplomat, there were two different speedometer pinon adapters, one for smaller tooth count pinons and one for larger tooth count pinons. I do not remember the cut off and am not sure it is relevant for you because it was definitely not the same type you are running.

But based on it only have one aliment mark labeled I gotta wonder if it might be so. If anyone has an 88 part manual they might be able to look it up and clarify.

I would think you could change the housing that has more alinement marks and #s and go with a gear bigger than a 25 tooth.

the little gear box inbetween the housing and cable sends the signal to the cluster.

should not matter what adaptor it is hooked to.IMO


you would adjust it to the correct alinement for the tooth count and hook the speed sensor up to it.

I will wager a 27 tooth count will get it right.

post tire size and gear for the math/chart.

one thing for sure if it is not adjusted correctly it will eat the plastic teeth right off of the pinion or skip and not turn. BTDT

So I have a 27 tooth gear (white) that I salvaged from an old 727 tranny. I have the speedo housing from that tranny as well with the different ranges. Will the 27 tooth speedo gear from a 727 fit an '88 904 tranny and function without destroying plastic speedo gear???

from looking at it,just swap in the other adaptor and gear from 727 and clock it in the correct # range and it should work with no problems.

recheck speed with GPS

Before changing pinions, have you checked the odometer against a known distance? In many cases the speedo error isn't because of the wrong pinion but is because the speedo head is faulty.

When checking for the correct pinion always go by the odometer, not the speedometer.

If you`re within 10 mph., each tooth change is equivelant to about 2.5 mph. Find a 28t.
And if you want to sell your speedo pinion gear housing, I`m interested. I`m running a 24t in my wagon.
Dean