well let use my trusty super numerical aerodynamics calculator. lets see its um R=Rho*v^2*A*Cd*0.5 so click click grind screech clunck chuug chug chug kerplow poof.
18mph increasing to 20mph is between a 23% and 23.5% increase in wind resistance depending on the exact drag coefficiant of the bike-rider combo.
Thats at sea level, if you want another elevation I can do it real quick but it won't vary much more than 0.2 percentage points