On both my dad's 92 Previa and on my 2011 Prius, I have noticed that when I am running the mix of Regane HM and gas, the engine operation is significantly smoother. Much smoother. As soon as the tank of treated fuel is used up, the smoothness is gone. The car performs the same as it did prior to the treatment. Can anyone speculate why this is happening? Should I get a stronger fuel additive, such as BG 44k?