Look what I found on another forum, it's a tahoe, but it should be the same concept...
"The only thing that takes a couple of tries to to re-calibrate the needles.
First time I did it wrong and when I filled up with gas it only showed 3/4 full and the speedo was showing 70mph when i was at about 55 mph.
Easiest way to recalibrate is before you put the needles back on you connect the wiring to the gauge WITHOUT turning on the ignition.
Once connected you hear the needles go back to their 0 position.
Then while it's connected push your needles on in the "0" position.
Then disconnect the wiring and turn all the needles to the 12 o clock position.
Then replug in the wiring into the back of the gauge and the needles should all return to the "0" position.
If they are not exact then repeat the process till they return to the "0" position.
Once they are correct then put the plastic cover back on and re-attach the gauge to your dash and put back the 4 screws.
Then put your dash bezel back and your good to go."