Gas Mileage???

brasil said
when I looked inside the cylinders.... the surface of the piston was not clean...so I can´t see anything like numbers.. or so...
the plugs are middle brown... btw

Carbon on the cylinder tops but sparky is clean. Interesting. Since what you're seeing is cumulative from all sorts of driving its probably not good to jump to any conclusions. However it is good to make a mental note of it.

The mixture @ idle is around 12.5-13 AFR

OK. (see AFR relation to load for ranges to expect). That doesn't change the fact that the engine seems to be telling you it wants slightly richer. As long as it doesn't load up the spark plugs with carbon, give it what it wants.
PS. Check it in Drive as well as Neutral - it might get a little leaner in D because the vacuum went down.
PSS. The AFR mixture from a WBO2 is based on Lambda and a factor based on an assumed stoichiometric ratio of 14.7:1. If the petrol used has a different density and stoich (as is common here in the USA), the actual afr will be a little different than what the meter shows. Our usual petrol here nowadays is something like 40% refined petroleum, and as much as 10% ethanol. :( Even racing fuels vary quite a bit when it comes to density and stoich. But at least the suppliers of race fuel publish all the specs...
Anyway, the point is not to target an AFR, but pay attention to what AFR gets the results.