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Speedometer Calibration -Techs Please

vwnut

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1999 Ranger 3 Ltr 5sp with RWABS
Changed tire size from 225 70 15 to 235 75 15
Also changed rear end from 7.5 to 8.8
From what I've read, tire size is changed in the GEM module
and the rear end size is changed in the RWABS module
I have access to dealer software and looking for some direction as to the procedure to make the changes.
 


RonD

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1998 to 2000 Rangers used the rear axle ABS sensor for speed, only years for that setup

RABS or 4WABS don't care about 7.5" or 8.8", so not an issue, ABS just watches for "sudden" change in pulse counts, i.e. lock ups, much like Cruise Control, neither has any idea of what "speed" is, they both just use pulse counts

Yes, the GEM Module, behind the radio, was used to calibrate speed signal in 1998-2000, again only years for this
The GEM just needs the "RPM" number, Rotations Per Mile, of the tires, you can look that up

Chart here: https://ntwonline.com/tire-shop-talk/revolutions-per-mile-chart/

Not sure which software was needed to "talk to" the GEM, but should be doable via OBD2 port
There may be a selection for axle size, as the tone wheel would be larger on a 8.8" so more pulses per mile
7.5" tone ring has 102 teeth
8.8" 108 teeth so 6% difference

Axle ratios don't matter here since you are using axle rotation directly, not drive shaft rotation, like most speed signals did and do, it was just these 3 years that did it this way

Again, it was only these 3 years of Rangers that used the GEM for this so fairly obscure even for Ford Techs, lol
The Explorers used 4WABS module for speed calibration during these 3 years, but not the Rangers, just FYI, in case you come across that
 
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superj

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wouldn't it be less pulses per mile if the ring is larger? same as putting larger tires on makes your speedo read slow?
 

RonD

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The ABS sensor has
102 pulses per axle rotation, 7.5"
108 pulses per axle rotation, 8.8"

Pulse per mile would depend on tire size at the end of that axle, the roll out distance on that tire in one revolution of the tire

Fords calibration for speedometer is 8,000PPM(pulse per mile)
The GEM(1998-2000) needs the pulse per axle rotation and the RPMs(rotation per mile) of the tire to convert ABS pulse to the 8,000ppm needed by speedometer


29" tire has 720 RPM at 108 pulse per axle rotation = 77,760 ppm
GEM converts that to 8,000ppm
 

vwnut

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So, if I understand you correctly, to accurately calibrate the speedometer to the new tire size, you would also have to make the change in the rear end size?
 

RonD

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No

GEM in 1998-2000 only needs RPM(revolutions per mile) of the tire, which is tire's diameter/size

GEM "Might" have a setting for rear axle size(102 tooth or 108tooth) but it won't make it too far off in any case, Ford's speedo's(like most car makers) allow for 5% +/- error at 60MPH
So at 60MPH you could be going 63 or 57MPH, lol
 

vwnut

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Thanks for clarifying that.
Thought I read somewhere axle size was changed in the RWABS module.
I wonder if you have to access that module directly or would it link through the GEM?
I see in the wiring schematic that the RWABS has a diagnostic port.
Looks to have a plug configuration different than the OBD2
 

rubydist

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In practice, the Ford speedometers will read fast with the factory size tires. In other words, the speedo will read actual speed + 0-5% I have never ever seen one read slow with the correct size tires.
 

Eddo Rogue

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Its much easier to make the adjustment in your head. I use the my phone gps speedo to calibrate.
 

RonD

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Explorers with 4WABS(4 wheel anti-lock brakes) used the 4WABS module, 1998-2000, instead of the GEM Module to calibrate speed signal
4WABS was standard in Explorers in those years

1998-2000 Rangers WITH 4WABS "could" use the same feature of the 4WABS module, and some may have, just never ran across it myself , it would need to be a different wiring harness, but just 1 wire changed

4WABS was an option in Rangers thru 2000, in 2001 it was a standard feature but OSS sensor was used for speed starting in 2001

RABS is completely different system and never had speed function available
 

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