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Chevrolet Trail Blazer speedometer variance


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I just drove my wife's Chevrolet Trail Blazer yesterday for few hours and noted that it has huge notable variance on the speedometer as per with two different mobile sat nav (Iphone and Sony).

At 120 km I was getting 110 in GPS, I understand the small variance is acceptable but this 10 km variance seems little big. In my Mustang I get 117-118 in same mobile GPS when I do 120 as per car speedometer.

Any idea how I can fix this variance....?

Let's root for each other & watch each other grow.

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Trailblazers are known for their speedometer problems. In most cases, the stepper motors that drive the instrument fail, and the most reliable remedy is to replace the cluster. However, the problem with this approach is that most used units will have the same problem.

One (temporary) cure is to disconnect the positive battery cable for a few minutes, but this is not something I would recommend on any vehicle, no matter what the problem is, since you can seriously damage the entire electrical system by doing this.

There are companies in the US that repair this problem, but this is clearly impractical for you, so the only advice I have is to search for a used unit in your market, and hope it does not have the same problem.

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Hi Gaurav, how are you doing?

It's worth checking your tire size isn't smaller than the factory fitted that might also result in such variance.

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Nearly all cars over read the speed on the dash by up to 10%, it's just one of those facts of life and not something I would worry about. It's inbuilt by the manufacturers to stop people unknowingly exceeding the speed limit then suing the manufacturer because they got fined for speeding. Depending on the car, there are sometimes kits that can be bought to modify the speed sensor if you're doing something like fitting a different size of wheel and tyre and the reading is going to be way out, usually just consists of changing the worm gear, if it's a mechanical sensor in the gearbox. However, a lot of new cars read the speed through the ABS sensors so I'm not sure how you would go about changing that. I guess if you were good with a computer and had the right software that would be the way forward.

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Thanks guys for all advices. It is important to see the right speed to avoid fines and not to block any traffic by driving too lower than road limit. Although it is not critical that I will sweat too much for this. Here is the tire size that I clicked, any idea is it same size or smaller than stock for 2009 Chevrolet TrailBlazer 4.2 Liter V6 Engine.

IMG_7350.jpg

Let's root for each other & watch each other grow.

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@Technician is right about all speedometers over reading a bit, but in the case of the Trailblazer and a lot of other GM products, the overeading is just the start of a problem that eventually causes the speedometer to stop working altogether. 

17 inch rims is about standard for all Trailblazers, and the 60 mm profile should not cause the large speed variance you found. If it were my vehicle, I would start looking for a replacement cluster, all the while hoping the replacement does not have the same problem.

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Hi Gaurav, as I suspected it is true that your vehicle is fitted with slightly smaller height tire that might be resulting in this speed discrepancy.

Your tire size: 255/60R17    29.0x10R17    29.0"
Stock tire size: 245/65R17    29.5x9.7R17    29.5"

However this half inch must be causing lack of 2 kms plus the over-reading by default. My car over reads by 4-5 kms anyways.

Capture.JPG

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