eather you have the wrong size tire or the speedometer driven gear is wrong,or some one has changed the gear ratio in the rear end. these are the only things that could cause your speedo to be off.
Added:
Take your car to a dealer who will tell you what speedo gear you should have. The gears are not expensive and easy to change. Should take about 10 minutes including all the jacking of the car. Your axle ratio is coded in the VIN plate.
Fast enough...
120.......160
crown victorias, pi, are fast especially with microchip installed, which regulates the gas and others components, now, impalas which also happen to be police imterceptors are a little faster,
you will need a power steering seal replace.
You probably have a bulb out. Remember, the rear has two blinker bulbs
It tells you how fast you are going.
Engine rev's higher, car does not move too fast ahead.
speedometer
A speedometer tells you how fast you are going in a car.
A speedometer tells you how fast you are travelling but an odometer tells you how far you have travelled.
Look at the speedometer.
it tells you how fast a car is going