speedometer/odometer off on 2011 Tahoe

[GoodFrequency]

Hey all,
New member, although I browsed here for a while while researching Tahoes.
I took delivery of my 2011 Tahoe yesterday and upon driving it have discovered that the speedometer is off by a considerable amount, especially at interstate speeds.
Using a GPS today, the speedo reads 5 mph faster at 35 mph, 10 faster at 50 and 15 faster at 70.

Consequently, the odometer is off as well, with the car saying I have covered 20 miles when actually I have only driven 17.

Has anyone else had this issue and is there an easy fix without resorting to a mechanic or dealership?

Any idea how costly a fix this would be if I took it in?

Thanks in advance!