1990 Seville Speedometer Problem

I have a 1990 Seville and I have a problem with the speedometer not registering the correct speed. it's the digital speedometer and it registers a speed appx 40 miles per hour more than i'm actually driving. it's also making the odometer turn faster. I've had the speed sensor replaced but that didn't solve the problem. My mechanic says no codes are coming up about the problem and she doesn't know what to do about it. any thoughts?