Just as an FYI.
Whenever you alter the OEM suspension/tire/wheel combo, you'll get similar results as I did below WITHOUT having your speedometer caliberated to offset the changes.
I used the Apple Application "Speedometer" and downloaded it onto my wife's iPhone 5, sat it up on the cluster and hit I-95 for a couple of miles to compare real-time.
I first tested it to see how it would compare with my Tahoe in terms of accuracy. Both my Tahoe [OEM] and the Apple App read identical between 45MPH - 80MPH.
Below are the pictures I snapped when I compared the Global Positioning System traced MPH, to my NON-CALIBERATED setup.
You can clearly see the diferences.
CC set at 55MPH
CC set at 60MPH
CC set at 65MPH
CC set at 70MPH
The above discrepancies are enough to get you a speeding ticket.
I purchased a Hypertech Speedometer Calibrator  this week [arrived today] and will install it tomorrow. The $150.00 or so for or the Hypertech Speedometer Calibrator is worth every penny IMHO as you'll pay that, and then probably some more, for a SINGLE speeding ticket.