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ABS Sensor/Speedo Calibration


Explorer Addict
May 8, 2000
Reaction score
City, State
Eastern NC
Year, Model & Trim Level
'11 Explorer
There is another thread where it is discussed about changing out the ABS sensor.

ABS Sensor

IT has been explained by others how the Speedo's work on 98+ models. Basically from what I understand it is off the ABS sensor.

ABS recalibration

So is it possible to just replace the ABS sensors with ones that are already set up for EX's with 16" rims and 265/70 tires?
It seems to make sense to me.

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More than likely, the sensor is the same regardless of what size tires are on the truck. All the sensor does is create pulses as a toothed ring (which is mounted to the differential) passes by. The number of pulses created would be the same for one revolution of any size tire. The number crunching done by the computer equates the number of pulses to a given tire size. This in turn would give the proper signal to the speedometer. For example, assume that the toothed ring has 100 teeth (to make the math easy). A 235/75-15 (29.0” dia) rotates 717 revs/mile. This would create 71,700 pulses/mile (717 revs/mile times 100 pulses/rev). A 31x10.50-15 (30.7” dia) rotates 676 revs/mile. This would create 67,600 pulses/mile. The computer is programmed to take the pulses/mile value and translate it to whatever output signal is required to give a reading of one mile per hour on the speedometer (1mA per 1 MPH of speed, for example).

The TruSpeed module probably takes the pulsed signal from the sensor and electronically converts it to a different signal to fool the computer into thinking that you still have the stock size tires. Following the above example, if you put 31’s on the truck, the TruSpeed module would would be calibrated to take the input of 67,600 pulses/mile and convert it to 71,700 pulses/mile, then send this signal to the computer. As far as the computer system is concerned, it would think that the truck still has the stock size tires.

Disclaimer: I do not know how much of this information is accurate as far as the Explorer is concerned, but it does (in my experience) give a general explanation of how rotational sensors function and how the data they create is processed.

That is exactly how I am assuming the system works. I am just wondering, is it the sensor that is manipulated, or the computer? From what I have heard, when Ford calibrates it, they alter the RWABS sensor. Since quite a few people have trouble finding a dealer that will alter the system for them, is it possible to alter it ourselves?

Thanks for the reply.

The best I can tell from the info I have, when a recalibration is done, what is being modified is the setting for the tire size in the 4WABS module. What they are changing is the constant value used by the computer to calculate the output signal going to the speedometer. In order to get the sensor to read differently, you would have to change the toothed ring to a ring with a different number of teeth.