Ring and pinion altering speedo?
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Senior User
Joined: Apr 2001
Posts: 223
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From: Canton, OH
Ring and pinion altering speedo?
Hello. I'm having my 3.73's swapped out to 4.56's Thursday (new 33" tires). My speedo is obviously off, but dude said (at 4 Wheel Parts in Parma OH) that when they do the work it will change the speedo back to nearly perfect.
Speedo sensor/controls are up front I thought?
96 F-150
Lemme know.
Thanks!
-Nick.
Speedo sensor/controls are up front I thought?
96 F-150
Lemme know.
Thanks!
-Nick.
Senior User
Joined: Jan 2002
Posts: 262
Likes: 0
Ring and pinion altering speedo?
[updated:LAST EDITED ON 25-Feb-02 AT 01:33 PM (EST)]The speedo sensor is located in the differential housing on my 95. If your truck is that way too I would think that changing the ring and pinion would alter its reading, but I don't know if it would change it to an accurate reading. I guess it might be accurate if you were effectively gearing back to the stock ratio. What I mean is the bigger tires effectively gear you higher, the 4.56s gear you back down, maybe if the new combination is similar to the effective gearing with stock tires and gears then the speedo will be close to dead on, I don't know just guessing. I'm just not sure how it really works. I myself would like more info on this subject if anyone knows.
-Cope
95 F-150 4x4
4.9L M5OD
Front Truetrac Rear Detroit
3.73 Precision Gears
Warn Premiums
4" Superlift Superunner
-Cope
95 F-150 4x4
4.9L M5OD
Front Truetrac Rear Detroit
3.73 Precision Gears
Warn Premiums
4" Superlift Superunner
Senior User
Joined: Apr 2001
Posts: 186
Likes: 0
From: Tigard+Corvallis OR
Ring and pinion altering speedo?
I have read that on the 92-96 modles that the sensor reads the rotation of the axle and not the driveshaft. This way changing the gears wont have an effect on the speedo, but changing to 33s would. I havent changed my gears yet, so I am not 100% sure on this, but I remeber reading it somewhere on a post on this board. Im sure somebody knows for sure
Senior User
Joined: Dec 2001
Posts: 304
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From: san diego usa
Ring and pinion altering speedo?
Your ring and pinion ratio have nothing to do with your speedometer. On trucks that have a sensor in the rear differential, the rotational speed of the ring gear is measured, not the pinion. The ring gear's speed is determined only by the speed of your tires. The diameter of your tires is the only thing that will affect your speedo.
Technically, you could change the tone wheel that's sanwiched between the carrier and the ring gear, and replace it with one that has a different tooth count, and that would allow you to change your speedo reading... but nobody ever does that.
Technically, you could change the tone wheel that's sanwiched between the carrier and the ring gear, and replace it with one that has a different tooth count, and that would allow you to change your speedo reading... but nobody ever does that.
Senior User
Joined: Jan 2002
Posts: 262
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Ring and pinion altering speedo?
Thanks for the info. Since the sensor is reading ring gear speed that makes sense. But if you have a truck where the speedo hooks up to the tranny and you do a ring and pinion swap but no tire change then the speedo will read incorrectly, right?
_Cope
95 F-150 4x4
4.9L M5OD
Front Truetrac Rear Detroit
3.73 Precision Gears
Warn Premiums
4" Superlift Superunner
_Cope
95 F-150 4x4
4.9L M5OD
Front Truetrac Rear Detroit
3.73 Precision Gears
Warn Premiums
4" Superlift Superunner
Senior User
Joined: Dec 2001
Posts: 304
Likes: 0
From: san diego usa
Ring and pinion altering speedo?
[updated:LAST EDITED ON 25-Feb-02 AT 11:54 PM (EST)]You are correct. 
It's nice to have a tranny speedo sensor, because if you use bigger tires and then make the appropriate gear change, your speedo will stay where it should.
It's nice to have a tranny speedo sensor, because if you use bigger tires and then make the appropriate gear change, your speedo will stay where it should.
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Post Fiend
Joined: Nov 2001
Posts: 7,698
Likes: 4
From: Neenah, Wisconsin
Ring and pinion altering speedo?
With the speed sensors that are in the diff. you can calibrate them for larger tires fairly easy. You are correct that they will not be off from a gear swap. Do a search for speedo calibration and you should find the article detailing it. I'm working on a table for giving the right calibration numbers,,,but it'll probably be a while till i have it done.
Ryan
Ryan
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