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Discussion Starter #1
How is the tachometer adjusted and/or calibrated on this model. Just changed rear end ratios and need to set tachometer. Some have said there are switch settings on back of tach. Others have said there is a spur gear on tach or transmission. Anyone know the real answer?
 

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nobody likes a quitter
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You need to adjust your Speedo, not your tach.
 

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The Oracle
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you dont need to adjust anything. the tach is not affected and niether is the speedometer.

the speed signal is produded by the tone wheel on the rear carrier. there are a set number of teeth on it, that does not change. the ONLY thing that effects the speedo on these trucks is the tire diameter, because they change the distance covered by one revolution.

so as long as you didnt change the tire size, your distance covered is the same regardless of the gear ratio..
 

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Discussion Starter #4
Now I am truly lost. After changing from 4.88 to 3.73, the speedometer reads correctly. The tachometer reads the same RPM at 65 MPH, that is 2500RPM at 65 MPH. So either the rear end ratio was not changed or the tach is not reading the correct RPM. I know the speedo is correct by my Garmin Nav system and interstate mile markers. The engine does sound/feel like it is turning fewer RPM. But tach still reads 2500 RPM. So something is wrong someplace!
 

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I changed gears in my f-450 from 4.88 to 4.10 an speedo was correct afterwards. just out of curiosity I unplugged the sensor on the rear diff housing and speedo still worked but the ABS light came on. I think the speedo pickup is in the trans.
 

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TennesseeHillbilly
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IIRC.....RPM reads off the CPS...and feeds back to the PCM....changing anything other than how far you push the go-pedal isn't gonna/shouldn't affect the tach.

BTW I have seen switch settings on the back of aftermarket tachs....to change for 4, 6, or 8 cylinder application.
 

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Freak'n know it all
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just a little fyi..... if your tach DID run off of the rear diff or anything that would be affected by rear end ratios you would show 0 rpm when idling and not moving.

The only explanation I can see working here is that your speedo was off with the old gears and the gear swap made it accurate.

of course that doesn't work with bigred97's tone wheel statement.
 

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Sounds like it's not dropping into OD.

Which makes me think:
If I were towing 5k# at 65 mph with 3:73 gears and od locked out 2500 rpm

And a similar truck with 4:30 or lower is towing same load same speed OD locked in and spinning the same rpm. Would it be similar towing power??? Would the lower gear run faster or slower?
 

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The Oracle
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I changed gears in my f-450 from 4.88 to 4.10 an speedo was correct afterwards. just out of curiosity I unplugged the sensor on the rear diff housing and speedo still worked but the ABS light came on. I think the speedo pickup is in the trans.
if signal is lost from the rear speed sensor, the abs computer will subsitute front wheel speed(when equiped with 4wasb) and your speedo will continue to operate

IIRC.....RPM reads off the CPS...and feeds back to the PCM....changing anything other than how far you push the go-pedal isn't gonna/shouldn't affect the tach.

BTW I have seen switch settings on the back of aftermarket tachs....to change for 4, 6, or 8 cylinder application.
exactly

just a little fyi..... if your tach DID run off of the rear diff or anything that would be affected by rear end ratios you would show 0 rpm when idling and not moving.

The only explanation I can see working here is that your speedo was off with the old gears and the gear swap made it accurate.

of course that doesn't work with bigred97's tone wheel statement.
only if the speed sensor was in the trans would gearing effect it...like an old foxbody or similar ford
 

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Discussion Starter #11
Thanks everyone. Mystery solved for now. It seems due to mislabeled packaging the wrong gears were installed. So correct gears on order to arrive next week and back into shop for installation. I am old school, find it hard to believe they didn't do a quick count of the teeth or at least compare side by side. Doing either would have saved some unnecessary work and expense as now they get to do it all again at no cost to me. Anyway, we shall see for certain next week. Also, did finally find a reference online which showed the tach does use the CPS as stated by one poster. Just found it hard to believe the shop could put the exact same gears back. Even though I did see the original's after the work was complete, since I wanted to check on wear.

The reason for the question on setting the tachometer, is that on some of the big trucks, at least according to those who should know, there are DIP switches which must be set according to gearing. Doesn't make sense to me and don't know how it works.
 

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Freak'n know it all
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Another good example tho of how inaccurate the SOTP dyno can be
The engine does sound/feel like it is turning fewer RPM.
It's amazing what we can feel/hear when we are specifically looking for it. :D
 

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TennesseeHillbilly
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Thanks everyone. Mystery solved for now. It seems due to mislabeled packaging the wrong gears were installed. So correct gears on order to arrive next week and back into shop for installation. I am old school, find it hard to believe they didn't do a quick count of the teeth or at least compare side by side. Doing either would have saved some unnecessary work and expense as now they get to do it all again at no cost to me. Anyway, we shall see for certain next week. Also, did finally find a reference online which showed the tach does use the CPS as stated by one poster. Just found it hard to believe the shop could put the exact same gears back. Even though I did see the original's after the work was complete, since I wanted to check on wear.

The reason for the question on setting the tachometer, is that on some of the big trucks, at least according to those who should know, there are DIP switches which must be set according to gearing. Doesn't make sense to me and don't know how it works.
Glad you solved it......I agree they should have looked....every gear I have ever installed has the number of teeth for the ring gear and pinion gear stamped on the outside diameter of the ring gear. Simple division will give you the ratio.......
 

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When I had my E99 550 I was told to forget about changing out the 4.88's.

Most shops told me it wasnt possible. One shop told me they did it once, and will never do it again.

I think the 4.88's were the smallest gears to fit all the "internals" of that differential... In other words you cant just change "gears" to a lower ratio.


Let me know how much you end up paying and how it works out, Im curious.
 

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Discussion Starter #15
The original estimate was $1500 which grew into $2500. Since it is a heavy duty commercial rear end with no differential cover plate they have to go in from the front drive line side, more labor. Then it was necessary to replace the carrier assembly in order to fit the new gears. So apparently it can be done. I say apparently as at this point can be certain until after next week when they will supposedly install the right gears! Then I will be able to post back with certainty.
If I had it to do over again might have simply gone with a Gear Vendor. Still if in fact I get the 3.73s and, as planned, keep the truck for 15 years or so, think I will be happier.
 

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Discussion Starter #16
Another good example tho of how inaccurate the SOTP dyno can be

It's amazing what we can feel/hear when we are specifically looking for it. :D
Yep. In my case had the Banks Stinger Plus installed at the same time. So could tell some difference in performance, both acceleration and cruising at speed, but certainly no significant, if any, change in RPM at 65 MPH. That coupled with very limited, as in almost none, of big truck tachometers which do have dip switches on the tachometer. So I thought maybe they had put different gears just not the 3.73s and maybe some 200 or 300 reduction in RPM.
 

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Administrator
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the original poster has not signed on in 9 years. so i really doubt you will get an answer.
 

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f550 has a spicer s135 rear end. the lowest gearset I could see what 4.63 . so ther eis no point in changing it. mine is a 4x4 and I have never heard of d60r gears in 4.63 to match that oddball. stuck with 4.88s in f550 from my searching. if there is something else available I am interested.
I assumed the speed sensor is on the carrier like the f350s (but I havent looked since I bought the truck a month ago) so a gear change would not effect the speedo.
 
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