I put this excel file together a while back after reading an article on Autospeed called
Real World Measurement of Drag and Lift. He had a calculation to measure Cd. I was more interested in reversing the calculation to get horsepower at a specific speed, so I made a calculator. The downside is that it does not take into account rolling resistance.
To use it, you will need the Cd of your vehicle, length and width (or just frontal area), and input a speed.
Horsepower at Speed Calculator