Calculate Rolling Resistance Experimentally

I am trying to calculate the exact rolling resistance ($F_{rr}$) of my car. The approach I am going with is a coastdown test at a low speed so no drag slows down the car. From there I am going to measure the speed of the car several times and find the acceleration and therefore $F_{rr}$. With the help of an online polynomial regression, I am going to find the function of $F_{rr}$.

However, I am not sure if my approach is correct and if the function that I am going to find is indeed capable to give me the $F_{rr}$ at higher speeds.