My car asks for 91 octane, but my choices around here are 87, 89, and 93. Octane is for knock resistance in higher-compression engines. I use 89 octane. All an insufficient octane will do in a modern engine is have the computer reduce your horsepower a bit.
Just so I understand, would filling the tank half-way with 89 and then topping off with 93 create something just as good as 91? I suppose knowing exactly how much to pump to fill the tank half way would be a problem. Any other problems with this scheme?
That’s pretty much how the gas pump does it. There may be three grades of octane at the pump but there’s usually only two tanks in the ground. When you select the middle octane rating the pump mixes the two together.
Another data point: with both my current and previous vehicles, whenever I go down south to visit my relatives, I drive thru the mountains in PA. Both vehicles would be badly underpowered going up the hills (especially when heavily loaded with people and luggage), until I hit on the idea of planning on running low, and filling up with premium, just before I got that far. I surmise that the underpowered condition was because the knock sensor was doing its thing properly, and robbing some power (in exchange for sparing some potential damage) just when I needed it most. Using premium cured the problem.
In fact, when I first went there with my current vehicle, I didn’t bother making sure I would be on empty when I got to northern PA, so I still had half a tank of regular. When I got to the hills, and the problem happened with the new vehicle, half a tank of premium at the first exit I came to worked well enough, so I could probably get away with 89 if my tank were near empty.