1996 GMC 2500 series - Speedometer reading fast

I got a 1996 2500 series Suburban and the speedometer is reading faster then I am actually driving. Using GPS to check my speedometer I found that it's reading about 15 mph faster. Don't like the idea of a speedometer showing I'm doing 85mph when I'm actually doing 70mph.. It has the original 454 & transmission & stock size tires 245/75/16 on it. I bought it from the original owner & was told it's all original & by looking under it looks like nothing has ever been replaced. Is there a way to correct this problem?...joe

Joe, GM had lots of gauge problems in the trucks of that era. The gauges are all electronic, each individual gauge has a little stepper motor on the circuit board, and those were prone to failure. There was even a recall back around 2007 or thereabouts to repair gauge problems based on year & miles. Google 1996 GM Gauge Problems, or 1996 GM Speedometer Repair to find a bunch of links to discuss the problems. Hope this helps you find your gremlin.