if your car (factory tune 93Oct.) is using 91 you may have some knock and not know it,, the 100 runs better because the car is (from the factory) set up for 93 you should MAYBE you could mix 100 4 gal to 8 gal of 93 to give you 94 OCT. and no knock and better mpg,, or 9 of the 91 and 3 of the 100 gives you the 93OCTand saves you money. jim