Im driving on 22inch rims with everything else mechanically factory standard , I fill it full tank to the top and always manage around 550 to 580 km combined urban and freeway kms using standard e85 ethonal fuel.
if i were to fill it full tank with 98 octane it manages around 600 to 640kms
for a heavy vehicle they are pretty good on fuel compared to other trucks this size using unleaded fuel

 

as doc olds mentioned to improve economy let it breath better with with an exhaust upgrade and if in you're budget re map the ecu . keep it finally tuned helps as well

 

When I first started tracking fuel economy on my 06 adventure with 33inch mud diggers, k&n air filter, roof rack and plus 1 light bar , I was getting 11~12 mpgs [winter time].

During the summer, I did a throttle body and maf cleaning, changed the spark plugs and fixed a p0305 cylinder 5 misfire issue. Since then, I've been getting 14~16 mpgs. Psi stays around 38 all season.

 

My '07 Adventure consistently got 17-18 MPG with the OEM 33" tires in mixed driving - local plus highway but often in traffic. Now with Goodyear Duratrac I am seeing 15.5-17 MPG in similar conditions. No modifications, just keep the MAF & TB regularly cleaned.