Speedometer is showing less than actual speed, help!

[UNDsioux]

Hi All,
Long time lurker, first time poster. I am having an issue with my speedometer, it reads slower than my actual speed. I sent the cluster off and had all of the motors repaired and calibrated with the correct tire size 265/70R17. I replaced the speed sensor located in the transfer case and still no luck. Below are some examples of what it's reading.
Speedometer is reading ///Actual Speed
20MPH----------------------------24MPH
37MPH----------------------------45MPH
48MPH----------------------------55MPH
54MPH----------------------------65MPH

Vehicle is a 2005 Tahoe Z71, 5.3L any ideas why the gauge is reading incorrectly?
Thanks!

 

[Slckls1]

did you change wheel/tire size? rear end gearing?

 

[UNDsioux]

Nope, original tire size. I am not original owner but I believe it's the stock rear end.

 

[Slckls1]

that's the only thing I can think of that would throw it off. I'm no expert, though!

 

[mdnitedrftr]

What are you using to read your actual speed?

 

[Tonyrodz]

Using a GPS to measure actual speed?

 

[UNDsioux]

Three different GPS speed apps, looking at the numbers it seems as if it's reading about 17% slower than actual speed. In past vehicles they have been off by about 3 mph at any speed. Could the previous owner install a tune that could change the accuracy?

 

[Rocket Man]

Three different GPS speed apps, looking at the numbers it seems as if it's reading about 17% slower than actual speed. In past vehicles they have been off by about 3 mph at any speed. Could the previous owner install a tune that could change the accuracy?

Yes, if he had the tune calibrate for different tires/wheels which he swapped out before he sold it. BB tuned my pcm for my 26" wheels, for instance.

 

[UNDsioux]

Yes, if he had the tune calibrate for different tires/wheels which he swapped out before he sold it. BB tuned my pcm for my 26" wheels, for instance.

Is there anyway to reset to factory without having a programmer?

 

[Rocket Man]

Is there anyway to reset to factory without having a programmer?

I don't know. I bought an Autocal so I have the original tune file that I can reinstall if needed. Most tunes are done differently though, since the Autocal is expensive. Plus there's many different tune companies and programmers. You should get ahold of a tuner like Black Bear and see if they can answer questions. They're very helpful.