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Replacing the Speedo Gear

February 21, 2018.

How many miles do you have on your truck? If you're anything like me, you'll answer this by looking at your odometer, reading a number, and concluding that you have that many miles - 83,078 miles in my case.

Like me, you're wrong.

Your odometer is lying to you. And so is your speedometer. I covered why that is here - go read it. I'll wait. Speedometers, Odometers, and Gas Mileage – All Lies!

OK, welcome back. Or not if you're like me and just skipped over that last link. Let me try to recap:

The reading from your speedometer and odometer are affected by lots of things - like tire size, differential gear ratio, and more. Because of this, car manufacturers make a "best effort" to get the speedometer to be "close" (usually a little bit fast) in a stock configuration. Unfortunately, I'm no longer running a stock truck and the 33" tires and 4.88 differential gears have resulted in my speedometer reading 80 mph when I'm actually doing 69 mph.

All that means that I don't actually have 83,078 miles on my truck. I've actually got about 75,000 miles on it. Not that I'm planning to sell it any time soon ever. But as much as I want to hit the 500,000 mile club, I would rather that my odometer be correct - or at least as close as possible.

To that end, I ordered the largest speedometer gear they make for a 2000 Toyota Tacoma - the 33-tooth model - to replace the 30-tooth model that was installed in the factory.

It arrived, along with a new gasket and clip and I promptly set it aside while @mrs.turbodb and I drove several thousand miles down, through, and back from Death Valley.

It was an epic trip. Don't miss reading about it here: Death Valley (Jan 2018).

Now, several weeks later, it's time to install that gear, and hope that my speedometer reads slower than I'm actually going, and my odometer logs fewer miles than I've actually gone. You know, so the resale value of the truck is higher.

The first step was to remove the skid plates, since the speedometer gear housing is attached to the transfer case, which is well-protected by the @relentlessfab mid-skid. Six bolts and a floor jack, and I lowered the skid to find about 5 lbs of Montana, Wyoming, and a bit of Utah dirt that had hitched a ride back from The De-Tour (another epic adventure you shouldn't miss).

The speedometer gear housing is secured to the transfer case with a single 10mm hex bolt, which was easily removed after I disconnected the plug for the sensor.

With the bolt removed, the housing can be pulled out of the transfer case easily (if the o-ring hasn't degraded) and mine came out with a rewarding sucking sound, along with a few teaspoons of oil.

Fingers crossed that losing a bit of oil isn't the end of the world here. At this point, pop out the spring clamp, and pull the old gear out of the housing. Again, you'll get a nice sucking sound as it comes out. It was rewarding enough that I did it a couple times .

In order to fit more teeth (thus rotating more slowly to bring the mph and odometer readings down), the new 33-tooth gear has to have a larger diameter than the old 30-tooth gear - which it clearly does.

As a bonus, it's also painted bright pink. And we all know, most ladies like pink. So if you install one of these, it's more reason for them to like your truck.

The new gear slides right into the housing and is secured with the newly supplied clip. I also replaced the o-ring, though my original still seemed just fine, and was ready to re-install the housing assembly.

Installation is simply the reverse of removal, and I found that the machining tolerances of the hole in the transfer case, and the speedometer gear housing were so close that it helped to wipe a bit of oil on the housing in order to get it to slide in (with a few gentle taps).

I then re-attached the bolt ("tight") and plugged in the sensor.

And then it was the moment of truth. Would my speedometer (and odometer) be accurate? There was only one way to find out - I started up the truck and headed out to I-5 to compare my GPS speed with my speedometer.

Alas, it wasn't correct - but it was a lot closer. It now reads about 6% high (instead of 14%), which matches what the truck originally shipped with from the factory.

So - at least on my truck, the following configurations result in nearly identical speedometer/odometer margins of error (~6%):

And, other configs I've run:

  • ~0% error
    • 255/85 R16 BFG KM2 tires + 4.10 gear ratio + 30-tooth speedometer gear
  • ~14% error
    • 255/85 R16 BFG KM2 tires + 4.88 gear ratio + 30-tooth speedometer gear
    • 255/85 R16 Cooper ST Maxx tires + 4.88 gear ratio + 30-tooth speedometer gear

 

So yeah, my odometer still lies, but I don't really care. Because it just makes the adventure "longer."

3 Comments

  1. Andrew
    Andrew November 29, 2018

    Damn glad I found this. I did a 4WD conversion on my 04 double cab prerunner this month. I used a 4Runner transfer case that did not use the speed sensor hole to determine speed so it was plugged. I popped a 33 tooth gear in and it's been a little slow. It will read 31 when I"m going 38. Hopefully going down to a 30 will fix it.

    • turbodb
      turbodb November 29, 2018

      Glad you found it too - hope that speedo gear works for you. IMO though, being a bit slow is better than a bit fast...means you're racking up ODO miles more slowly too, hahahahaha!

      • Andrew
        Andrew December 10, 2018

        30 tooth gear count was perfect for my 265/75/16 tires + 4:10 geared truck. Maybe about half a mph off. Thanks again.

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