it's also illegal because it makes the mileage increase. if it's not working, the mileage on your car won't go up. but the only way anyone will know is if they're sitting in the car with you. in other words, if you get pulled over, they can't tell!
as far as I can remember, it's illegal to "doctor" a milometer ie. turn the numbers back and then sell it with ungenuine miles. Trading Standards would come down like a ton of bricks on salesman doing that. Don't know if it's in the Big Book of Law, though.