We have all heard that the faster you drive the worse your fuel economy is. Just what is this relationship? A 2012 study measured the fuel economy (in miles per gallon) based on the speed of the car. The results are described below:

Speed: 40 50 60 70 80

mpg: 33.2 31.9 27.9 24.1 20.5

These values give the following sums: x = 300 x²= 19000

y = 137.6 y²= 3899.32 xy = 7924

What is the correlation coefficient?