I have a 2013 Tundra, base model, doublecab, 4WD, tow package, 4.6 L, 6 speed, and still running OEM-size tires. I can get the rated mileage if I want to, but that is with light loads, easy foot on the go pedal, and staying at 70 mph or below on the highway. Have even hit 20 mpg on occasion. At 75 mpg 18 mpg is more likely. Cold weather at 75 will drop it to around 17mpg.
If you run oversize and/or heavy lugged tires you'll lose gas mileage. A heavy mud/snow tire can take 10-20% off gas mileage. If you travel 75+ mph or spend a lot of time in mountains your gas mileage will suffer. If you have a heavy foot and/or brake excessively yada yada yada. I would expect that if you cruise open highways out west at 75-80 mph your gas mileage will suck. In summary - the mileage depends on your driving and optional equipment.
I also had a 2005 Tundra doublecab with the 4.7 L and 5 speed. Gas mileage for me was just about identical under same driving conditions.
Towing is a whole other ball game. A light, open, low profile, single axle trailer with a total weight below 2000 lbs might drop mileage from 19 down to 16/17 mpg. A high profile, single axle enclosed trailer around 3000-3500 lbs can drop it to 10-14 mpg highway.
I don't pay as much attention to local driving mileage. When I've bothered to calculate its been in the 14-16 mpg range.
Last edited by MikeL2; 01/17/19.
