Topic: Speedometer and Odometer Calibration

[Jeffy]

Part I: Overview

As many of you know, changing tire size or gears will affect the speedometer and odometer reading.  This is because the gauges have been calibrated for the stock tires and stock gear ratio.

Here is a simple formula for those who have changed their tire size;

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Old Tire Dia. x Old Speedometer Tooth Count / New Tire Dia. = New Speedometer Tooth Count
Now if you've changed the gears but have kept the same tires use this formula;

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Old Speedometer Tooth Count x New Gear Ratio / Old Gear Ratio = New Speedometer Tooth Count
So what do you do if you've changed both the tire size and the gear ratio?  Well, you use a chart like this;

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Gear Ratio/ Tire Size 27" 28" 29" 30" 31" 32" 33" 34" 35" 36" 37" 38"

                 5.38                                                           42      41      40     39     38     37     36
                 5.13                         42 41 40 39 38 37    36    35
                 4.88                         41 40 39 38 37 36    34    33
                 4.56                 42 40 39 38 37 36 35 34    33    32
                 4.11 42 41 40 39 38 37 36 35 34 33 32 31
                 3.73 37 36 35 34 33 32 31 30 29
                 3.54 36 35 34 33 32 31 30 29 28
                 3.07 31 30 29 28 27 26 25 24 23

Keep in mind that variables such as actual tire diameter vary the tooth count.  The chart should get you within 3-5mph.  Using a GPS to verify, you should be able to get with in 1mph.  Just to give you an idea, I've gone through 4 different speedometer gears since buying my Jeep new.



Between 1987 and Present Day (2005), there have been a few changes with the VSS or Vehicle Speed Sensor.  The earlier speed sensors used a cable.  This ended in 1990 when MPFI was introduced.  Then in 1993 there was a switch from the long 4.5 inch speedometer gear to the shorter 2.2 inch.  You should be able to identify these by the shorter VSS housing.

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Short shaft part numbers (1993-2006)

- 52067629 - 29 tooth speedo gear
- 52067630 - 30 tooth speedo gear
- 52067631 - 31 tooth speedo gear
- 52067632 - 32 tooth speedo gear
- 52067633 - 33 tooth speedo gear
- 52067634 - 34 tooth speedo gear
- 52067635 - 35 tooth speedo gear
- 52067636 - 36 tooth speedo gear
- 52067637 - 37 tooth speedo gear
- 52067639 - 39 tooth speedo gear
- 52067640 - 40 tooth speedo gear
- 52067641 - 41 tooth speedo gear   

I've noticed many places having the same long shaft parts numbers.  These are wrong.  The 35T and the 39T I received have different numbers and do use the tooth count as the last numbers.  Since have two gears, I was able to piece together all of the other parts numbers.  Notice that 38T isn't listed on other websites but is available.

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Long shaft part numbers (1991-1993)

- 52068153 - 29 tooth speedo gear
- 52068154 - 30 tooth speedo gear
- 52068155 - 33 tooth speedo gear
- 52068156 - 34 tooth speedo gear
- 52068157 - 35 tooth speedo gear
- 52068158 - 36 tooth speedo gear
- 52068159 - 37 tooth speedo gear
- 52068061 - 39 tooth speedo gear
- 52068062 - 40 tooth speedo gear
- 2538941 - 41 tooth speedo gear
- 2538942 - 42 tooth speedo gear   

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Long shaft part numbers (1987-1990)

- 53001492 - 29 tooth speedo gear
- 53001488 - 41 tooth speedo gear
- J3212729 - 29 tooth speedo gear
- J3212730 - 30 tooth speedo gear
- J3212731 - 31 tooth speedo gear
- J3212733 - 33 tooth speedo gear
- J3212734 - 34 tooth speedo gear
- J3212735 - 35 tooth speedo gear
- J3212739 - 39 tooth speedo gear
- J3212740 - 40 tooth speedo gear

Part II: Disassembly

This is an easy job that anyone should be able to accomplish in a few minutes.  All that is required is a 13mm wrench or socket.  First the harness needs to be disconnected from the VSS.  (*on pre-90 this would be the speedometer cable.)  There is one 13mm bolt holding the clamp in place.  With the bolt removed the clamp and VSS should pull straight out.  Be sure to take some care in removal, there is an O-ring that can be damaged.  If it's an older Jeep the O-ring could have dry rot and fall into the transfer case.  Fishing out the pieces would be a chore at best.

   

(*notice in Pict 4, the worm gear on the output shaft is just visible.)



With the VSS removed, look at either the side of the housing or the base, you will notice numbers.  These correspond to the tooth count of the speedometer pinion gear.  The clamp will need to be repositioned if the tooth count is beyond the range set.  The two tabs on the clamp will sit in the grooves on the VSS.  Between them will be the number range corresponding to the tooth count of the speedometer gear.

The reason for this is to account for different sizes in the pinion gears.  There are three different sizes that correspond to the three different range settings.  The shaft of the VSS is offset so as it is rotated the gear is moved towards or away from the worm gear on the output shaft.  If the VSS is offset too far, the speedometer will not work.  This sometimes takes more than one try to get set up.



While the VSS is out, apply some oil to the O-ring is it looks try.  If the O-ring is damaged or missing, ATF will spray out of the gap.  An O-ring is cheap so you might want to get one when you pick up the speedometer gear.


(*the red gear is the stock 41T.  The white is the new 37T and the orange is the 35T.)

Once it's all set, tighten the clamp and reconnect the harness or cable.

Part III: Why Recalibrate?

Well, there is the obvious, to have a accurate speedometer and odometer.  But there is a lesser known reason.  The PCM looks at the signal from the VSS and uses it to help set the Air/Fuel Mixture & Timing.  This is important since you will feel a larger drop in performance and not know about it.

Some sites recommend that it's better to have the speedometer read faster then slow, so you, as a driver, can be sure you're driving over the speed limit.  I think this causes the PCM to think you're cruising when you're actually struggling.  What is probably happening is the engine is running a bit lean because of the difference in Speed vs. RPM.

Since changing to the 35T, my speedometer has been reading 4mph fast  (at)  65mph for a while.  This isn't normally a problem but it does seem a bit sluggish.  When I went to the 37T, the torque shot up a bit and I could easily feel the difference.  So, I'd say it was a good investment.