First tank, I got 720 km on about 40 litres. That works out to 18 km/litre, or about 40 US MPG, or 5.55 litres/100km.
Second tank, I went 735 km on 41.6 litres. Midway through this tank I changed out the air filter.
Third tank was mid-grade gas (89 octane RON/MON average) and I went 745 km on 41.07 litres.
So mileage is good... how to make it better? I have some lowering springs for the front (already installed some on the rear, dropped the car about 5/8 in.). Lowering the car may cut the wind resistance profile a little, and since most of my commute is done at 80 to 100 km/h that's going to matter. Synthetic lubes will start going in as I do maintenance (current odo reading is about 60,000 km) and from past experience with the Tundra that may make 5% difference. I'll keep posting mods/mileage as things develop...