408lq9

after having car tuned, drove it around and noticed that the mph gauge is off. when i'm doing about 45mph, it shows 95-100 mph on the speedo. i thought it was just off because of tuning. took the car to the track and it would run to the 1/8 and get on the rev. i thought it was running threw the convertor. i put a new convertor and had the trans rebuild and it does the same thing. its a 408ci with a turbo 400 trans, and its not spinning the tires. could this be a tuning issue.