Speedometer Reads 3% Off

Not2oldyet??

Newbie
Original poster
Sep 1, 2022
2
Clearlake Oaks CA
My 2005 GMC Sierra WT came with 245/70 R17 tires and i have always used this size. The instrument cluster was replaced by the Dealer when the stepper motors started failing. There was an optional (larger) 265/70 R17 tire available. I have wondered if the replacement instrument cluster was calibrated for the larger tire ?? Is there a way to make an adjustment so the speedometer is accurate. Yes, I could change the size of the tire, but wanted to know if it can be changed.

Just joined due to my recent "Check Engine' light codes and was impressed with the diagnostic assistance to others. Will be doing my research and testing in the next few days
 

Mooseman

Moderator
Dec 4, 2011
23,693
Ottawa, ON
The calibration for the speedo is in the PCM, not the cluster. On the GMT800, the PCM should be programmable with tuning software like HPTuners to adjust the speedo calibration in the PCM.

Another option may be to find a same truck year with the same gearing and tire size in a pick-a-part yard and put that PCM in your truck, which would require only a security relearn.
 

Joe_67

Registered Member
Aug 9, 2022
56
Central Virginia
My speedo reads off (under by about 3mph) from oversized tires by about 3mph. I just, well, know that. And adjust my brain accordingly. And double-check myself with any manner of GPS-based phone app once in a while.

But sure, like Mooseman said, you can get tuning softwares to mess with it.
 

Dr.Fiero

Registered Member
Dec 7, 2017
139
S. Alberta
IF it's 3% off across the board (like, 10-100mph or whatever) then... chances are that when the stepper got replaced, someone simply didn't stick the needle back on the pin properly!

Just take the cluster apart, hold the shaft with a hemostat or something, then VERY carefully remove it, turn it that degree or two left or right as needed and push back down. Do yourself a favour and test drive it before reassembly.
 
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