Speedometer correction after gear swap?

[Bamtech1]

After changing from 4.10 to 3.54, plus 285/75/16 tires my speedometer is way off. Like 15 mph off.
If I’m thinking right, you can just change the pinion gear on the speedometer sensor, but I’m having trouble finding that info. Found a table on a Jeep forum where you could plug in axle ratio and tire size to find which gear to buy. That was for XJs though.
Anyone have any experience with this? So far I’ve been riding with an Iphone speedo app.

Thanks!

 

[GAmes]

You do know that all 2nd gens are not the same, right? If it is a '97 or older it is simple math. If you are traveling 60 mph and the speedo is showing 45 divide 45 by 60 = 75% Count the number of teeth on the gear. For a simple example 32 X .75 is 24. You need a gear with 24 teeth.

 

[KATOOM]

Without knowing what year or model truck you have, I dont know if you installed a new carrier or just new R&P. I cant remember the carrier split and I didnt feel like looking it up.
Either way, both should have utilized the original tone ring. That said, you're going to have to reprogram for the new axle speed in order to have the speedo read accurate. Example - https://www.turbodieselregister.com/Issues/Web_Tip_DodgeSpeedometerCorr.pdf

 

[Bamtech1]

My apologies, i didn’t provide enough info. I guess I need to figure out the signature feature. It’s a ‘96 2500 4x4 with NV4500 and 241 tcase. Just replaced both r&p along with new Truetrac rear carrier.
So, GAmes, you’re saying do the math, then subtract from the existing gear? I’ll check that out, thanks!

 

[GAmes]

There isn't any subtraction. Count the number of teeth on the gear you have and do the math.

 

[Signal73]

I thought on my 96 the speedo pickup is on the transfer case. I had to switch the sending unit and worm gear on the output shaft to make up the 15mph difference.

 

[GAmes]

I thought on my 96 the speedo pickup is on the transfer case. I had to switch the sending unit and worm gear on the output shaft to make up the 15mph difference.

On 4X4s it is on the TC, on 2WD it is on the tailshaft. You don't have to replace the sending unit or the worm gear, only the round plastic gear. Be sure to clock the assy as described in the FSM.

 

[Signal73]

That sounds about right, it’s been more then a few years!

 

[GAmes]

I haven't worked on one since my G56 conversion 9 years ago.

 

[Bamtech1]

Thanks much, y’all. We were driving home most of the day today and checking offset at various speeds, they all came up to .8 ish. Finally got home and pulled the sender, the present gear is a 36T. So by your example I’m looking for a 29.

 

[GAmes]

That should do it if the speedo is showing slower than actual speed. You have never really said, only that it is off.

 

[Bamtech1]

That should do it if the speedo is showing slower than actual speed. You have never really said, only that it is off.

Sorry, my communication skills are lacking. (That’s what my wife says!) I’m bad about assuming you CAN read my mind.

Yes, it reads low on the dash compared to GPS. The roundest number example would be at 60mph on the cluster GPS says 75.

 

[Bamtech1]

The new gear came today, I just had to go stick it in and go for a ride...
Success! Just a short honk, but the dial looks dead even with GPS at 60.
Funny, I thought when I ordered from LA Speedometer Gear it would ship from commiefornia. Nope. Shreveport.

 

[GAmes]

I love it when a plan comes together.

 

[Bamtech1]

Thanks again for the help. We’ll test it out and get some new mpg numbers on the trip to Cedar Key next week.

 

[Bamtech1]

One more question if I may:

There’s a raised pointer at the 6 o’clock position on the speedometer drive bore. Should the notch in the speedo drive line up with this mark, or should the hold-down crab sit in those notches? It works both ways, I actually like it a little better twisted a little further with the crab out of the notch. Just a little less gear lash. I didn’t find the FSM mentioned above. Funny what I did find.

 

[GAmes]

The FSM shows how to clock the sender assy. It depends on the tooth count of the gear. The hold down feet fit into the base.

 

[BigPapa]

"LA" doesn't always mean Los Angeles....

We have our own "LA" here in Nashville, Lower Antioch.

 

[Bamtech1]

They used to call the local junior college here UCLA.
Upper Canton, Lower Acworth

 

[BSeyler]

On the Wind River Indian Reservation, LA stand for Lower Arapahoe.