Speedometer Gear Selection

[Falcon_LS]

I'm due for new tires on my '99 Eddie Bauer (4.0L SOHC V6/5R55E with 3.73 rear gears), and 255/70R16 is no longer a size I can acquire locally.

After much reading on this forum, I decided to step up to 265/75R16s and ordered a set of four BFGoodrich tires. The only issue is obtaining the correct speedometer gear.

I have looked at various sources to help select the right one, including The Ranger Station, this post and this post, which was the inspiration for this. One of the main posters upgraded to 265/75R16 tires on his Explorer, also with the 5 speed automatic, but with a 4.10 gears and decided the 19 tooth (CODZ-17271-B/Pink) was the way to go.

I'm not sure which speedometer gear I have, and unfortunately, I'm not in a position right now to crawl under the truck, due to back issues. To save the wait, I will most likely end up having it done for me at this point.

I picked the next way out and checked the Ford Microcat catalog by VIN. It claims basic part number "17271" is not applicable to my vehicle, which further complicates things. I also checked various online parts catalogs, such as Giant Ford Parts and Ford Parts, and had no luck there either.

I was wondering if anyone happened to know which speedometer gear a 5R55E/3.73 combination would come with. Any input would be greatly appreciated, thank you!

 

[donalds]

On my 99 the sensor is in the rear diff
I can change the tires to any size
then use my Sct x4 to calibrate for the new tire size

 

[Falcon_LS]

Thanks for the input!

If it's a matter of changing the configuration in the GEM (or the ABS module as I have been reading), I'll have a go at it with my Autel. Seems '98+ does not have a speedometer gear based on what I've been reading, but uses ABS sensors.

 

[410Fortune]

correct
the cable went away in 95
the whole vss sensor (the old gear driven ones you could change out the gears) went away in 98
In a 99 sploder the rear axle vss sends a signal to the ABS module that does the buffering, that signal is then sent to the GEM, Cluster, cruise control and more

If you run tires too large the software corrections can no longer adjust for the tire change
In this case I use a speedometer interface to adjust the buffered signal from the ABS computer to the rest of the truck
With the dakota digital BT-1000 I you can adjust the speedo as you drive with bluetooth even with 32" tires or larger

 

[MakinJeepsWeep!]

Yup my 96 still the little gear. Got mine withing 1 mph. It reads 1 fast so it keeps you safe putting cruise on the exact speed.

 

[Falcon_LS]

Awesome, thank you very much for all the input!

In the interest of keeping things simple, would the software be capable of accommodating a 5% difference?

I'm really more concerned about transmission shift points than speedometer accuracy. I guess essentially, I will be "dropping" from a 3.73 rear axle ratio to 3.55 with the change in tire size. Will have a go at it on the Autel as soon as I get a chance.

Thanks again!

 

[donalds]

Awesome, thank you very much for all the input!

In the interest of keeping things simple, would the software be capable of accommodating a 5% difference?

I'm really more concerned about transmission shift points than speedometer accuracy. I guess essentially, I will be "dropping" from a 3.73 rear axle ratio to 3.55 with the change in tire size. Will have a go at it on the Autel as soon as I get a chance.

Thanks again!

I will look when the sun comes up

I can adjust shift firmness
Shift points at WOT
and tire size
I'll make a vid for ya if you want

 

[410Fortune]

Yup my 96 still the little gear. Got mine withing 1 mph. It reads 1 fast so it keeps you safe putting cruise on the exact speed.

Well sorda, it maybe 1 mph fast at a certain speed...... but its off by a percentage and that percentage changes as we climb up and down the speedo, so it maybe off by 1 mph at 60 but when you get to 80 it will be off by a larger value

@donalds MAKE THE VIDEO!!

 

[donalds]

Well sorda, it maybe 1 mph fast at a certain speed...... but its off by a percentage and that percentage changes as we climb up and down the speedo, so it maybe off by 1 mph at 60 but when you get to 80 it will be off by a larger value

@donalds MAKE THE VIDEO!!

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[imp]

Awesome, thank you very much for all the input!

In the interest of keeping things simple, would the software be capable of accommodating a 5% difference?

I'm really more concerned about transmission shift points than speedometer accuracy. I guess essentially, I will be "dropping" from a 3.73 rear axle ratio to 3.55 with the change in tire size. Will have a go at it on the Autel as soon as I get a chance.

Thanks again!

The ages old system used two speedometer gears, one on the transmission output shaft, turning at driveshaft (pinion gear) speed, called the "Drive Gear", the other on the cable leading up to the speedometer head, this gear called "driven gear".

Invariably, the drive gear had less teeth than the driven. LONG ago, it was removable from the transmission output shaft, and could be replaced with another of differing tooth count. Midway, maybe 1980s, the drive gear was machined into the shaft, making change far more difficult, plus few choices were available (seem to remember 7 or 9 teeth).

Stepping up to 4:11s or 4:56s in my early days made speedometer calibration an interesting feat. Ford supplied driven gears with up to 20 or 21 teeth, which sometimes were not quite enough. More than 21 was impossible, given the diameter of the gear. Almost all were tough plastic, rather than metal. Confounding the issue, I learned the driven gears had to be chosen for "hand"; right or left hand rotation, depending on where they were mounted on the tailhousing, left side bottom, left top, or right side.

Fortunately, my boss at work had been employed in a speedometer shop! He explained something which helped immensely. ALL speedometers used in America operated at a cable speed of 1000 rpm to indicate 60 mph! So, knowing the tire rolling diameter (rotations per mile) told me how to arrive at the gear ratio giving 1000 cable rpm @ 60 mph. I got most of mine within 1 or 2%, acceptable pretty much.

 

[Flashflood]

Awesome, thank you very much for all the input!

In the interest of keeping things simple, would the software be capable of accommodating a 5% difference?

I'm really more concerned about transmission shift points than speedometer accuracy. I guess essentially, I will be "dropping" from a 3.73 rear axle ratio to 3.55 with the change in tire size. Will have a go at it on the Autel as soon as I get a chance.

Thanks again!

Why you going down may I ask?

 

[410Fortune]

Fortunately, my boss at work had been employed in a speedometer shop! He explained something which helped immensely. ALL speedometers used in America operated at a cable speed of 1000 rpm to indicate 60 mph! So, knowing the tire rolling diameter (rotations per mile) told me how to arrive at the gear ratio giving 1000 cable rpm @ 60 mph. I got most of mine within 1 or 2%, acceptable pretty much.

Very interesting! 1000RPM = 60 mph, I had no idea
Ford speedos want to see 8000 ppm for calibration. Meaning 1 mile = 8000 pulses from the VSS, at least up until 2007, I have not worked on anything newer then that. With all the complex wheel speed sensors and the ABS computer (or gem on non abs models) doing the buffering the signal sent to the actual instrument cluster is still 8000 pulses per mile, just like back in 1983 when the ranger was created.
Once I learned that it made it possible for me to do the math and calibrate our OBD2 speedos using the dakota interface

 

[Falcon_LS]

Thank you very much for all the input, and special thanks to Donalds for that video!

I had no luck with the Autel; it prompts for the VIN under the Service tab, and then "fails" to decode the VIN when you select the Change Tire Size option. It "decodes" the VIN just fine for every other function, but I guess it just doesn't have the capability to make alterations on this particular vehicle.

Looks like I'm going to have to invest in an SCT X4 to make any changes. My SCT is VIN locked to my Grand Marquis, so it serves no purpose on the Explorer other than a code reader. I got an Autel and an AutoEnginuity scan tool with the Enhanced Ford option for diagnostics. Luckily, it doesn't get driven much and can wait until the X4 arrives.

Why you going down may I ask?

The rear axle ratio remains the same, but the overall diameter of the tire changes by 5%, which in turn reduces the number of revs per mile. As a result, there will be a decrease in the number of revolutions to turn the wheel a complete 360 degrees.

 

[imp]

Very interesting! 1000RPM = 60 mph, I had no idea
Ford speedos want to see 8000 ppm for calibration. Meaning 1 mile = 8000 pulses from the VSS, at least up until 2007, I have not worked on anything newer then that. With all the complex wheel speed sensors and the ABS computer (or gem on non abs models) doing the buffering the signal sent to the actual instrument cluster is still 8000 pulses per mile, just like back in 1983 when the ranger was created.
Once I learned that it made it possible for me to do the math and calibrate our OBD2 speedos using the dakota interface

All things in the automotive world seem to have become more complex! But, pistons in cylinders with valves controlling fuel/exhaust flow are pretty much the same.

Several things you must know which I do not:
Some interim period Ford drove the old-fashioned speedometer head, analog, using a motor driven off a speed sensor.....when?

What's a "dakota" anyway?

 

[410Fortune]

https://www.dakotadigital.com/pdf/sgi-100BT.pdf

This is how I calibrate speedometers on these trucks with large tires, and also how I interface with speedometers when doing v8 conversions....... the 02+ rangers do not like our 96-01 5.0 PCM's and speed sensors

 

[MakinJeepsWeep!]

Well sorda, it maybe 1 mph fast at a certain speed...... but its off by a percentage and that percentage changes as we climb up and down the speedo, so it maybe off by 1 mph at 60 but when you get to 80 it will be off by a larger value

@donalds MAKE THE VIDEO!!

Yes, without a doubt this is fact. I just rarely run my over 80mph. Also i do not know why (assume rear tanny ect gearing) the x does not seem to get as much of a sway is speed as my B3000 does, the B3 gets way off faster i go, both have 31-10.50-15's but x is awd auto B3 is 2WD 5 speed.

 

[410Fortune]

X = auto trans = power sucker
X = AWD = heat maker power sucker front end part abuser
X = 4800-5200# curb weight

B3 = 5 speed RWD, that little truck will keep accelerating till you reach the moon

Neither vehicle should be operated at speeds over 75 mph!!!!!!!!!!!!!!!!!!!!!!!!!!!!! LOL LOL LOL well maybe if you are down in Texas where it takes a 90 mile straight 5 lane highway just to get to the next town