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Discussion Starter #1
So, I am the second owner of my 2006 Colorado and ever since I have owned it the speedometer has been off. WAY off. It starts reading incorrectly as low as 25 mph and gets worse as it goes up. So bad that when the speedometer reaches 100 and the governor kicks on, the truck is only going 80 mph. I was told VSS, so I replaced it to no success, then I was told the stepper motor is bad in the cluster, replace it too, still same problem. Now the speedometer only works when it wants to, but still shuts the truck off at 80. I am tired of being shut off at highway speeds here in the great city of Memphis where everyone is always trying to qualify for a race and I am getting run over. I know a tune can kill the governor and help alleviate the over 80 situation, but it wont help the speedometer being so far off. I am thinking someone down the line did a gear change. Does anyone else have any other thoughts to why it would be doing this? I want this crap fixed ASAP. Thanks for any help, in advance!
 

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Discussion Starter #5
Nothing else wrong. However the tire plackard in the door shows all wheels on the truck being 15" options yet my truck came with the 17" wheel on it. Factory size is 225/75r15 (28.28" tall) and I have 235/50r17 on it (26.25" tall). I still think it has to do with the rear end, though. a smaller tire should not throw off a speedometer 20 mph! The truck is a 3.5L/4L60e, rebuilt trans with upgraded parts, CAI, magnaflow HF catalytic converter, gutted manifold, magnaflow muffler, and it has been lowered 3" in the rear using blocks. No CEL or pending codes at this time
 

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Discussion Starter #7
according to the calculator the speedo should be off 8 mph at 100, not 20. That is what makes me think the original owner put a weird ass rear gear in this thing
 

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I don't know how to make it so you can drive 100mph, but yeah there is something going on there.

The tire does change by around 8 mph at 100. Somebody would have had to change the rear end to a higher gear ratio, and who knows why they would do that. 2 things in my mind, fuel economy, or they had a rear end go bad and that was what they reinstalled as a repair. Not sure where an owner would have gotten a weird ass gear ratio as there are only a few to choose from.

How are you confirming your actual speed relative to dash needle?
 

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I do not see it being off that much without some help. I think a new cluster, and a tune, allowing for whatever gear is in the rear end, and tire size, would fix the problem, unless it is a problem in the computer itself. Being off 20 mile per hour is almost impossible with gear choices available, and the differences you stated in tire size. Just my opinion, nothing else.
 

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tinja nurtles
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Best bet is to find a person with hptuners. Pay the man and driver him around until he gets the speedo correct
 

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Maybe when the previous owner changed tires and wheels he had it adjusted and they may have gotten it wrong and he left it that way. As was stated find someone with HP Tuners and have them fix it plus at the same time they can tune it for you also.
 

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Discussion Starter #12
I am not saying I want to fly around at 100 mph, but when most interstate speed limits are 70 mph and your vehicle shuts down at 81, people are always running up on me, and I look like an anchor in traffic. I only mentioned hp tuners because a friend of mine a few hours away runs HP tuners and has offered to help with it, but it is hard to meet up with him when he is so far away and works 3rd shift. And he wants 200 bucks, and money isn't something I have a lot of laying around. I know that tuning it would most likely solve my problems, as I have been around drag racing and playing with cars all my life. Thanks for all the input guys. I have replaced everything but the cluster so I am planning on getting a full set from US speedo when I can afford it. The girlfriend just lost her job, so things are REALLY tight right now.....
 

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Discussion Starter #13
I used a GPS speedometer and a scan tool that measured at the VSS how fast the vehicle was going. When the governor hit and shut it down both registered 80-81 mph
 

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Discussion Starter #14
The only thing I could think was maybe a dad had this for his son and had someone go in and set something in the computer that shut it down at 80? But that still doesn't explain why the discrepancy was in the gauge itself, with separate stepper motors? It has driven me crazy ever since I got this truck!
 

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As far as I know, HP tuners (or similar) would have to be used to control the rev limit. For the expense, and void of warranty it's less likely that the previous owner tuned it down (but not impossible).

Your scantool should read the computer.....so if your needle is at 80, scangauge should read 80 also (I do believe). Was that true for you? I think that's what you said, but I got confused.

I don't know what a stepper motor does so I can't help you with that.

I don't remember the process to identify the rear end gear ratio. I thought it was to count the number of the tire revolutions relative to one turn of the drive shaft. Jack up the rear and put it in neutral (after securing the front wheels from rolling). Spin the tire around one complete revolution and take note of the drive shaft revolutions. Somebody correct me, but there is a way to determine by a method similar to that.

Compare that to what the gear ratio of what the truck was equipped with from the factory, then you'll know if the rear has been swapped.
 

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maniac mechanic
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To check the gears make a line from the pinion yoke to the rear end housing and make a mark on your tire. Have someone spin the tire one revolution while you watch the yoke. If the yoke spins 3.73 times you have 3.73 if it spins about 3 and a half you have 3.42 if it spins over 4 you have 4.10.

My guess is someone changed the tires and probably put a different rear end in to get a g80, and changed ratios in the process. Other than someone messing with the programming a tire and gear change are the only things capable of throwing the speedo off 20mph without a vss issue. Once you do the visual checks I said above, open the glovebox and the codes on the sticker in there will tell you what your truck is supposed to have from the factory so you can compare.
 

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I thought that was how it worked but it didn't count out properly for me when I tried it. It's been a long time since I've done it, I probably did something incorrectly.
 

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tinja nurtles
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If both are in the air one will typically spin backwards and the shaft won't spin at all. So when you put one tire on the ground and through the beauty of spider gears you have to spin it twice as far.

Spin the shaft and count wheel revolutions
 
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