I'm trying to calculate skin friction drag on a truck so I can compare it to pressure drag. I can find the formula and typical coefficients for pressure drag easily enough but I've had a hard time finding it for skin friction. So far I've found this: https://www.physicsforums.com/showthread.php?t=100705" Which gives the equation: Rf = 0.5 x (rho) x V^2 x S x Cf rho = density of fluid V= velocity S = surface area Cf = coefficient of friction I don't know what a plausible coefficient of friction would be, I found a calculator http://adg.stanford.edu/aa241/drag/SkinFrictionCalc.html" that I think is meant for aircraft, plugging in zero altitude and a plausible surface area (few hundred m^2) and speed (mach .09) I got .0027. Trying that out I get: .5(1.3)(29^2)(300)(.0027) = 443 at 65 mph whereas for pressure drag I get 1/2(1.3)(29^2)(9)(.75) = 3690 Which gives skin drag being ~12% of the combined drag. Is that right? I have no idea whether I'm entering a plausible Cf variable in the first equation, or doing it right. Can anybody help me with this? Thanks.