Looking to switch my tune from 89 to 93. How empty does my tank need to be before i fill up again to safely run the tune. I know the lower it is the better but is there a magic cut off point?

 

No but you could always just go to the gas station and get a 5 gallon gas can full of 93, then run your car until its dry, even it if means driving around the neighborhood 128974 times.. then when it runs out pull out the 5 gallon tank of 93 then drive to the gas station and finish filling her up.. then at least you know its as empty as possible.

 

I had a full tank of 87 octane when I did my Brenspeed tune, so I set it for 87 octane. I ran the tank down to empty by the fuel gauge, and then just filled with 93 octane and went on my way. It took at most 20 miles or so to begin to notice the difference, and its a pretty big difference! My mileage went up at the same time, so its all good.