Annoying VW Diesel...

I got a ScanGauge 2 that can display spark advance. I want to compare both 85 and 87 octane fuels to see if there's a difference in spark advance. I was going to wait until next spring when the weather warms up, but I might try this experiment this winter and see if it helps get me any better fuel mileage. I was getting over 40mpg average, but now I'm right at 30mpg and no better for the past three tanks. 30mpg is what I got in my 1972 VW Beetle in the winter! And I never could seem to get less than 45mpg in the winter in the diesel Golf! I have a hard time thinking it's the octane though that's causing my fuel mileage problem in the Avalon. If anything, colder weather should cause a less of a need for higher octane.