You guys with -9's are posting impressive numbers, you might have to form your own club. :-) As to the question as to what is going on with my -10, it might be a few knots faster than average, but I suspect that the most important things to note are the operational parameters.
In the post above, I am at a density altitude of 17,000 ft., and it seems that the unique airfoil of the RV-10 loves altitude. I am guessing that this has something to do with air density, Reynolds numbers, and laminar flow, but someone smarter than me can confirm this. Here is a plot showing segments of different cross-country flights, with cruise altitude plotted against fuel efficiency for each one:
These are flights at different configurations of power, RPM, fuel flow, etc., but even so, it is altitude that has the overwhelming effect on efficiency. And higher is better, at the rate of one mile per gallon for each 3000 ft.
The other operational aspect to note is the low prop RPM. I am running old fashioned Slicks at 25 deg. fixed timing, and 30-40 degrees lean of peak. As several excellent threads here have explained, the flame front needs time to do its work at high altitude and lean mixtures. Here is a plot from a test in which I did mixture sweeps at RPM's ranging from 2400 to 2000. Superimposed on the plot are isopleths for nautical miles per gallon. Test conditions were 9200 density altitude, so maximum efficiency is only about 18 nmpg.
There is a lot to be learned from this graph, but the simple point is that the maximum efficiency was at the slowest RPM tested, 2000. Here is another recent graph, showing the results of a test in which I simply reduced RPM in a stepwise manner while on a cross country flight (14,000' DA), without touching anything else.
X-axis is RPM, y-axis is true airspeed divided by fuel flow (nmpg).
So, fly high, and for those of us with magnetos, keep that prop slow.