all i have ever owned is chevys. there are so many advantages of going with a z-71 over the toyota. (toyotas are great trucks, don't get me wrong.) the gas mileage shouldn't be what is going to make this decision for you. look at the real factors involved: power, off-road ability, towing-ability, comfort, room, price, etc. you need to choose a truck that fits as many of the variables for you as possible. then make your choice.
people like to say gas mileage is a huge factor, but is it really? look at these numbers.
z-71 - vortec 5300 v8 5.3l - 26 gallon tank - 15/19mpg
tundra - v8 4.7l - 26 gallon tank - 16/18mpg
z-71 - 390 miles per tank
tundra - 416 miles per tank
tank of gas for each is $58.50 @ $2.25/gal (average). say you drive 1000 miles/month (average).
z-71 - 31 tanks/year or $1813.50 annual gas bill
tundra - 29 tanks/year or $1696.50 annual gas bill
cost difference of $117/year or $9.75/month.
of course, the specs of each vehicle were retrieved from their respective websites. the gas price is just an average, as well as, the annual mileage. don't let gas be the determining factor in a purchase of this magnitude. hth!