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Mechanical Speedo Correction Box - Install with Pics

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Old 11-04-2009, 09:31 PM   #1
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Mechanical Speedo Correction Box - Install with Pics

Not much of an install.

But, I have never seen pictures of these things before on YotaTech, so, since I bought one, I took some pics.

The box is pretty simple, it is a small gear box that you put in line between the transfer case and the speedo cable. I have 5.29's in my rig with 33" tires. This puts my speedo off by about 13%. The gear in my transfer case is the 18 tooth, so, no chance of changing that out, as they only make a small number of gears for it, and 18 is the biggest.

The box that I bought is a .879 reduction unit. Overdrive for the speedo cable. That is as close to .87 (13% reduction) as the gears available for the reduction box allowed.

I bought it from Commercial Speedometer in Sacramento, CA. Chuck at Commercial Speedo was very helpful and he turned it around in a day. I ordered it yesterday, on my doorstep today. Total cost was $95 with shipping and sales tax.

Picture of the reduction box:



Picture of the reduction box and the stock speedo gear module that I removed from the transfer case:



Picture of the two screwed together:



Picture of the unit installed:




I took it for a drive on the freeway, works great. There is a construction zone near my house that has a speed check radar sign at the start of the construction zone. I was going 60, it said 59. Good to go.

Last edited by OutlawMike; 11-05-2009 at 08:16 AM.
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Old 11-04-2009, 09:37 PM   #2
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Wow, that's pretty impressive actually. I thought I'd just have to live with my speedometer being off by over 26% for the rest of my life. I'll have to look into this product.
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Old 11-05-2009, 11:01 AM   #3
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Thanks for posting this, I knew there had to be something out there for the correction.
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Old 11-05-2009, 11:02 AM   #4
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neat
did he set it up for your specific tire size?
or is it adjustable?
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Old 11-05-2009, 11:58 AM   #5
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Stupid work computers, I can't see the pics.

I remember calling a speed shop once and asked them about getting a new gear for mine. It was gonna cost me some coin to get it fixed. In the end though, my tires/gears make my speedo dead on anyways.

I did not know that there was a mechanical reduction box(as opposed to the electical once like the Superlift Truespeed) for speedos, very cool.
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Old 11-05-2009, 12:32 PM   #6
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Quote:
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neat
did he set it up for your specific tire size?
or is it adjustable?
He does not do the math, so to speak. You have to tell him how much reduction you want and he will build it as close to that number as he can get, with the available gears that go into the box. In my case, I needed 13%reduction. He was able to give me 12% reduction with the closest gear combination for the unit.

To determine the reduction needed, you need to time your car over a known distance course at a constant speed. Or, pace another car that you are pretty sure has an accurate speedo and then do the math.

In my case, over a known 1 mile distance, driving at 75 mph on the speedo took 55 seconds. Converting my 1mi/55 seconds to mi/hr: 1mi / 55 sec *3600 sec/1 hr = 65.45 mi /hr. Since my speedo read 75 mph, this is a difference of 9.55 mi/hr. 9.55 mi/hr is 12.7% of 75 mph, which is the reduction percentage I needed.

It is adjustable, if you have him swap different gears in. I should have asked him for a table of what ratios he could hit.

A neat little solution, but not my idea in the least. The thanks goes to: Roger at 4Crawler. Of course Roger has already done this in the past and has recommended it in many posts on this and other websites. I just got off my ass and called around to find out where I could get one.

Thanks again Roger,

Mike
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Old 11-05-2009, 12:39 PM   #7
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Would this work for the left drop xcases on 3rd gen runners and tacos?
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Old 11-05-2009, 01:13 PM   #8
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A GPS also works well for checking speed. A friend's Garmin and my DeLorme agreed at constant highway speeds.
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Old 11-05-2009, 01:18 PM   #9
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Would this work for the left drop xcases on 3rd gen runners and tacos?
I'm pretty sure those use electronic vehicle speed sensors, but don't quote me on that. For those with an VSS, you would use one of the magic boxes out there that modifies the signal to the speedo.
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Old 11-05-2009, 01:32 PM   #10
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Cool, never heard of these before.
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Old 11-05-2009, 01:47 PM   #11
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Quote:
Originally Posted by BigBluePile View Post
...I did not know that there was a mechanical reduction box(as opposed to the electical once like the Superlift Truespeed) for speedos, very cool...
Quote:
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Would this work for the left drop xcases on 3rd gen runners and tacos?
IIRC, like stated all ready, yours is a electric pickup and you'll have to wire in a sensor box.
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Old 11-05-2009, 01:48 PM   #12
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Yup. Thought so =(
Wasn't sure where the elec pick up was done.
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Old 07-16-2013, 09:18 AM   #13
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Speedo Adapt to compensate for Mods

I contacted Commercial Speedometer today for a gear reduction because with oversized tires, speedo was incorrect. The man there knew exactly what was needed. $80.00 + shipping. This should keep the cops off of me. Thanks for the posting here. 916-371-5873. Literally a 5 minute call.
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Old 07-16-2013, 10:43 AM   #14
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That is very cool. Thanks for sharing.
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