You might be able to do something with energy. Your total energy at the top of the hill with be 1/2mV^2+mgh. Thats the kinetic plus potential energy, where m is your mass in kg, V is you speed in m/s, g is 9.8m/s^2 and h is the change in altitude in meters. To find the speed on flat ground, take the total energy at the top of the hill (E) and plug it into sqrt(2E/m) to get your speed. The problem with this is it makes a lot of assumptions that are not particularly accurate. It assumes your speed is constant going up the hill, and it neglects wind resistance, which increases with the square of velocity.