i want to build something like that, but i have no time and no room.

that is a nice vehicle.

 

as far as gas mileage goes, weight, gearing, drivetrain efficiency, and drag are the biggest factors i think. I got 30mph with my 5.7L blue-pill with lean cruise enabled at about 55 mph. High 20's shouldn't be out of the ordinary for any LS-powered car.

my 4.3L vortec blazer-wannabe struggles to get 20 on the highway. it's also as areodynamic as the side of a barndoor. i often wonder why i can't get better mileage, but see how high the average MPG's shoot up when i draft a semi (yeah, i know i shouldn't be riding this close...) and that confirms my suspicions. it's because it's a massive lego block with wheels.

 

This is overly simplistic, but think of the cylinder as a jar. As the size of the jar increases, to fill it takes more air and more fuel, assuming the air/fuel ratio is the same in both jars. Weight of the vehicle, trans and gearing play a role as well.

 

A 6.0 is only 12% larger than a 5.3, but many of them use proportionately more gas, and in most of those is because they are-
a) pushing a heavier 3/4 ton truck,
b) operating with lower (higher numeric) rear gears (like 3.73 or 4.11), and
c) running a 4L80 trans that uses more power

 

This is what I suggest. Priced well, stock stall friendly, will make a LOT more power than a stocker and if tuned up well should also improve fuel mileage. Best of all you can use the LS6/3 springs and keep your stock pushrods although Id suggest checking preload first and order new sized to fit moly pushrods. New valve seals and real delco LS7 lifters, rollmaster single chain ( i believe Summit brand are also rollmaster or at least they were ) or cloyes single also excellent chain. Maybe a fresh oil pump, etc. No need to break the bank but check things out and replace as nec.

https://www.summitracing.com/int/par...8719/overview/

 

6.0's don't get bad mileage, the vehicle they are usually in does though.

A 05 vette with an ls2 has no problem getting 30 mpg on the highway.

 

My wife was due for another daily driver. She wanted V8, stick shift and something fun to drive that we could haul 4 people comfortably. She bought an 06 GTO with the LS2 and T56. So far we rebuilt the suspension and lowered it slightly, full Kooks exhaust, 1.8 full roller rockers, Duspeed CAI, Fast 102 intake, Nick Williams 103 and it was dyno tuned. The GTO is very similar in weight to my S10 and it gets 13-14 in town but on the freeway at 75 we're pulling down 24-25mpg.

 

Keep in mind that the GTO does have aerodynamics on its side.

 

This is why I bought a CTS-V. The 6.0 in the trucks get so poor mpg mostly due to the safe factory tuning. Overly rich, overly retarded and 4.10 gears doesn't help. I bested 13 stock with my Lq4, and ended up with 16-18 after lots of tuning. No engine mods. In real world the new trucks get about the same. I don't know where the EPA gets their numbers from. Haven't seen 22mpg on anything except the V6. The Ford gets about the same with all its extra engine technology. It just takes a certain amount of power to move a big heavy brick down the road and power is fuel. I would say we are at about the extent of what we can do to a gas engine to increase its efficiency. VVT, DOD, OHC, efficient factory heads and exhaust, intakes, direct injection. How much have we gained from all that? 2 or 3 mpg? I'm not impressed. Power though is another story.......

 

6.0 LS2 498 to the tire I'm still getting 23 hwy

 

What trans and gears?

 

2006 corvette mn6

 

It's pretty clear that fuel economy-wise, it is more about the vehicle than the engine, engine being a 6.0 in all cases.
In a heavy, wind-pushing, low-geared, safely-rich tuned 3/4 ton truck, mileage WILL suck.
In a light, aerodynamically smooth, tall-geared and finely tuned Vette or GTO, mid to high twenties are likely.
Nothing unexpected here....

 

My 2019 silverado averages 22-23 with mixed city driving and 28mpg on the highway. (4 door, 5.3 1500)