Speedometer is totally off!

[HRV-LS1]

Hello,

Installed a 5.3 Chevy engine into my 95 GMC truck. I have 265/70/R17 tire wheel combination. My pcm was programmed but the speedometer is off. For example, when the speedometer is showing 40mph, I'm actually driving at 48mph. I believe that my rear differential is stock from the factory. I'm not the original owner, and I don't know for sure if I do have a stock factory rear diff. If its not, what rear diff might I have with my speedometer being off 8 mph?

 

[IADC43]

Following. Mine is wacked out too.
I wish somebody would come up with a GPS signal sender and interface for OEM speedos.

 

[95Suburban]

Dakota digital sgi-5 I believe. Will take any type of speed signal and convert it to any type of output, including GPS to oem. I used it for my Cummins conversion, converted Chrysler speed signal to gm. Also can fine tune actual speed output for different tires ,gears,etc. Around$80.

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