Calculate the number 3976

A fellow bought a new Mercedes and was out on the interstate for a nice evening drive. The top was down, the breeze was blowing through what was left of his hair and he decided to open her up. As the needle jumped up to 80 mph, he suddenly saw flashing red and blue lights behind him. "There's no way they can catch a Mercedes," he thought to himself and opened her up further. The needle hit 90, 100.... Then the reality of the situation hit him. "What am I doing?" he thought and pulled over. The cop came up to him, took his license without a word and examined it and the car. "It's been a long day, this is the end of my shift and it's Friday the 13th. I don't feel like more paperwork, so if you can give me an excuse for your driving that I haven't heard before, you can go."
The guy thinks for a second and says, "Last week my wife ran off with a cop. I was afraid you were trying to give her back!"
"Have a nice weekend," said the officer.