if a car goes to the quarter mile mark in 13.6 seconds, how fast will the speedometer show as the car's speed at the quarter mile mark?

 

I don't know what the speedometer says but it works out to 66.17645 MPH.

 

EDIT: the speed could be a little higher based on the weight of the car, any performance modifications and the fact that you are still accelerating when you went through the traps.

 

Also different tires size,

I know from experience when I could not find tires in my size anymore or limited selection I had to change the tire size.
Used a calculation to ensure the speedometer would not be off significantly. Tire Rack helped me out.

 

Does anyone have a GPS you could borrow?