car mileage, page-3

Wouldn't get too excited yet.
When you say around 510/520 a tank full, I would imagine that is a heap of stop/start driving, as opposed to a long trip.
Only way to get a true indication is to do the same length trip, preferably with the same weather conditions and volume of traffic, with each of the different fuels.

Then you'd probably need to do that at least three times to get a reasonable average.

Longer, country trips will always give you better fuel economy than city driving.