I've always wondered about some really high numbers like increase HP and torque which is great. But I've always wondered how they do long term and warranty wise.

I've boosted mine up 90 horse, so I'm probably close to the 400 mark. I don't hot rod around. If you go too much higher, you need to get some transmission work done. The tranny will go before the motor does. Check out some of the diesel forums, you'd be surprised out how much hp some are running on their daily drivers.

I'm sure the mfgs will not warranty a "programmed" engine. I know on mine, I can hook the programmer up and "return to stock" tune if needed be.

I have an 08 Tundra Crewmax 4x4 w/ 5.7 and over the last 4,000 or so miles I am averaging 14.8 mpg for everything. I know because I have to keep track for work. This includes highway, around town, towing (750 miles of the 4,000), and my lead foot. I have been consistantly averaging around 18-19 on the highway and around 13-14 in town when not pulling. The 20mph on the highway is probably very easy to achieve if you are not as heavy footed as I am and driving a 2wd.

You can have a tune or a programmer. I had Edge Juice and had 120hp. My neighbor stacked his Edge Juice hot and A bully dog program and dyno'd at almost 500hp at the wheel, with a few other mods. I think it was like 700hp at the crank. He put a 100K miles on it and never had a problem, but he did have a Suncoast stage IV tranny. His truck ran high 12's.

I have a 08 Tundra Double Cab with the 5.7L V8 and I drive about 50K miles a year with my job. I have a Snug Top bed cover on mine and I get 20.6 with my cruise locked at 70. I have seen 21 if I lock my cruise at 60 mph. The 4.7L does not get as mileage as the 5.7 because the 5.7L shifts more.

I must be in one of those moods today. Come on and be realistic. When a guy asks for an mpg he wants reality not the 100 mile stretch on the freeway with the wind at your back. A 5.7 gas engine is not averaging 20 mpg consistently, especially not with a 4.3 rear end.

I have a 6.0L with 4.10 rear end and I can hit mid 20's for spurts with active fuel management. I average 15.9 all around with the 6.0L Vortec Max. Put a 2500lb plus boat on any 5.3 - 6.0 gas engine from any make and you will be hitting single digits up to mid teens depending on how you drive.