'99 Tahoe 2WD Rear Gearing ?

[Ironhorse]

My new to me 2 WD '99 Tahoe has the option codes of GU6 and GU80 on the glove box RPO sticker. From looking on the 'net it codes as the GU6 being a 3:42 gear set and the GU80 codes as a '' locking differential '' or Posi-trac rear end.

I have the original size tires for the Tahoe per the door code decal which are 235/75/15s. Running down the road the speedometer needle which is rock solid with no vibration or movement shows per GPS phone app I am running 10 mph SLOWER than the indicated speed. When the speedometer shows 45 by my GPS app I am doing closer to 35?

Anyone have some thoughts to share? Has my rear gear set been changed? If so, what is the most economical way to rectify the situation to align the indicated speed and the true ground speed. Having just purchased this Tahoe I want to get things correct but do not want to spend a TON of money doing so, LOL.