Rearend Swap - Speed Sensor wiring

Discussion in 'Drivetrain Tech' started by Broncoholic, Feb 15, 2019.

  1. Broncoholic

    Broncoholic New Member

    I'm swapping in a full width 9.75 axle into my sons 99 Ranger. I'm curious if the wire harness off the Ranger can adapt to the speed sensor on the 9.75 and work the speedo correctly?
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  3. DeanMk

    DeanMk Member

    Are the connections even compatible?
  4. OP

    Broncoholic New Member

    Both sensors are the same size although I cannot get one of the sensors out to count the teeth.
    One sensor gives a 1.7Ohm and the other is 1.8Ohm. I assume this will cause the gage to be off.
  5. DeanMk

    DeanMk Member

    It could, but how would changing the wirinig harness correct this?
    I would think you'd have to change out the PROM at the ECM or swap out the actual geared wheel in the rear end that the sensor reads, for the one that would correspond with the gear ratio the truck originally had.....or maybe, you could change the tires to a size that would correct the change in gear ratio? <-- could be that simple.
  6. OP

    Broncoholic New Member

    I changed the gear ratio to 4:88 and will be adding 35” tires.
    Are you thinking I can swap out the speed sensor out of the 8.8 and put it in the 9.75?

    The wire plug harness off the Ranger is the same size by looks and plugs in perfectly to the 9.75 axle.
  7. DeanMk

    DeanMk Member

    I'm suggesting that you can possibly make up for any differences you may run across between what the speedo tells you and how fast you're actually going, by changing the size of the lieu of making a change to the computer itself.

    All you'd have to do is figure out what size to change to....and there is a formula for that....
    Speed in mph, times effective gear ratio, times 336, divided by the height of the tire.
    What you'll end up with is engine rpm for a given speed, gear ratio and tire height.
    So how does that help you?
    Since all the pertinent information is there, all you have to do is make a small mod to the formula and change some numbers.
    So let's say, your son's truck had 4.11's, manual trans. and 31" tires.
    What size tires do you change to, to keep things square?
    First we need to figure out effective gear ratio. Chances are good that your son's truck is actually an automatic, but I can't remember the OD ratio for that one right now and my truck is a manual, so I'm just going with what I know. Please feel free to input corrected figures in your own calculations.
    So, 4.11 x .79 = 3.2469:1 This is your effective final drive ratio when the tranny is in overdrive.
    Now we need to pick a speed. Since we're concerning ourselves with highway travel, let's just go with the national speed limit (60 mph). Again, please feel free to input corrected figures in your own calculations.
    Now we can run the formula....
    60 x 3.2469 = 194.814
    194.814 x 336 = 65,457.504
    65457.504 / 31 = 2111.532387096774
    That is the engine rpm when the truck is going 60 mph.
    Save that number, you're going to need it.
    Clear the calculator and run the formula again, but this time, input the final drive ratio for the NEW axle....
    4.88 x .79 = 3.8552:1
    60 x 3.8552 = 231.312
    231.312 x 336 = 77720.832
    At this point, you would divide that number by the height of the tire in inches, but since we're trying to determine a new tire height, we're going to divide by the engine rpm instead, so....
    77720.832 / 2111.532387096774 = 36.80778588807786
    That is the height of the tire required to correct the speedo for the change in gear ratio.
    ...if its easier, you don't necessarily have to calculate in the od ratio if you don't want to. Everything should still work out the same, you'll just have to deal with higher engine rpm.
    So there you go.
    If you need to calculate the height of the tires your son's truck currently wears and the height isn't noted in the size of the tire (like "35x12.50-15"), you can use the size listed on the tire to calculate height.
    So let's say the tires on your son's truck are "P235/75-R15".
    Just convert the tire width to inches (divide by 25.4), multiply by the aspect ratio (and that's a percentage!), multiply by 2 and ADD in the wheel diameter.
    Sidewall height is multiplied by 2 because the sidewall is measured twice when you measure the tire's height.
    Good luck.

    Last edited: Feb 23, 2019

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