Lets assume you have 10 gallon tank you add 7 gallons of 91 to 3 gallons of 100 you would get 93.7 Octane. However you will be running better than a straight 93.7 octane straight from the pump (there is no such thing as 93.7 octane pump ofcourse) becuase that 100 Octane has soo much hydrocarbons it increase the quality of this 93.7 Octane
Most people fill up 3.5 gallon of 91 octane and 1.5 gallon of 100 octane to get 5 gallons total, afterall more fuel you carry more weight on your car
