Here are some stats for you...
FWIW.... here are some numbers for you to look at and ponder, along with narrative for context.

op2:
My rig is a 2000 SR5 with Sport trim (driving lights, Limited style fender flares, etc). It is a 4WD, 3.4L Automatic.
The drive is relatively flat here, and no AC being used this time of year in Phoenix. The gas here in AZ is an ethanol blend if memory serves me correctly. I also have a 3" lift, no aftermarket roof rack, and until last week (12/27) was running stock size tires (265/70/16).
I drive approximately 80% highway miles and 20% city/surface streets in the time period shown below. Of the 80% highway, about half is early morning, no traffic @ about 75 MPH (40% of overall) and the other half is stop and go commute @ ~ 40 MPH (40% of overall).
My commute is about 35 miles each way. I am driving more than one vehicle, and spread the commute around, so don't worry that the mileage does not add up to 350 per week or more.
Hope this helps someone in one way or another...