I always use 87 octane and my mileage is all over the place. I was really worried after one road trip when the VX was about 4 MPG lower than usual. The next day I took a shorter highway trip and it was suddenly 2 MPG higher than average. On the way back it was 2 MPG lower than average. Now I keep an eye on wind speed and direction - apparently it makes quite a bit of difference!