Incorrect Spedometer Reading

[VTFrenchy]

Hello Everyone, I have done quite a bit of reading on this site, and gained some great info about my truck and trucks in general, but this is the first question I have not been able to find the answer to. I purchased my '09 Sierra about two months ago, and everything has been great, but when I was passing one of the police signs that shows the speed you are going, my spedometer was quite a bit off. The sign said I was going 36 and the spedometer said I was going 40. From reading earlier posts, I know that there is some kind of computer that can be adjusted for the size of the tires. My truck came with 265 70 R17s. Is this the typical size tire for the Sierra, it is a regular cab work truck. Has this happened to anyone else with a new truck? 10% difference in speed means that my odometer is also incorrect, which means that my truck has less milage than the odometer is reading. I also noticed that I am not getting the fuel mileage I am supposed to be getting, but I have read here that is typical for a new truck. Thanks for reading and any help provided.

Tyler

[bill190]

Take it to a GM dealer and have them check the settings in the "programming".

[georgecdisc]

Do you have a GPS by chance, they typically output current speed. Good way to check if your speedo is off. I wouldn't put much faith in those radar signs, but who knows, your truck might be incorrect. Can you get someone to drive their car going 35 and see if you go 35 also?

 

Edit: Also, could be a bad gauge I suppose. Maybe your truck (computer) is reading correct speed, but the gauge just isn't replicating it correctly.

[VTFrenchy]

Thanks for the replys guys. No GPS, but I am scheduled to have my truck have its first scheduled check up, so hopefully I will get it figured out. I will try having someone drive at a certain speed and see how close we are.

[brims]

Thanks for the replys guys. No GPS, but I am scheduled to have my truck have its first scheduled check up, so hopefully I will get it figured out. I will try having someone drive at a certain speed and see how close we are.

 

I don't know if you have them, but here in California sometimes they paint white + signs on the ground, they are supposed to be exactly 1 mile apart. If you have something like that you could time it and check.

[bubba33]

I have a gps unit and mine is off by about 2-3 mph.Truck says 60 and gps says 57 or 58. I have factory 18in tires but I doubt that has anything to do with it.

[J-man67]

In my experiences, I have never had a speedometer be spot on. They will always be slightly off.

[georgecdisc]

Comparing to GPS, I would say mine is off about 1 mph, maximum of 2 mph.

[txab]

Mine was off 2 mph until my tune.

[gmowner4life]

I noticed mine was off about 2 when I drove to TX in March compared to GPS.

[Mabe]

Most cars dont have an accurate speedo. Police cars do and they get calibrated every X number of miles. Almost every car or truck that I've owned has read 2-3mph high.