Have an 02 suburban I recently dropped a 6.0 into,I also had the eco flashed for the 6.0 motor,my problem is my gas mileage is horrible and I'm not exactly sure why? My check engine light has not been on so I believe no codes have been thrown,and the 6.0 is bone stock just rebuilt...any help is greatly appreciated

 

Don't know what to tell you. Perhaps the 5.3 liter or a 350 CI would have brought you some better gas mileage, but I can't say for certain. The 6.0 liter is a high performance motor, so its going to use more fuel.

A 6.0 liter motor is 366 Cubic Inches. In fact, Chevrolet produced a big block 366 CI motor back in the 1960's and 1970's that was scaled down from the 427 CI motor. It was only found in the medium duty trucks of the day. My father owned such a Chevy vehicle. At best it would pull 7 MPG, and that with a Holley four barrel carburetor and a point type ignition system. It had less HP the the 427 CI motor, but a better torque curve and durability then the 427, which ate gasoline like crazy.

You have to figure that if you put a big cubic inch motor in any vehicle, its going to use more fuel. In fact, I can recall that some Chevy pickups and Blazers of the time used about the same kind of gas mileage as my old man's truck did, which came with a 5 speed stick and a two speed electric-shift axle, allowing you to split shift it into ten speeds.

I obtained another MPG or so when I switched out the distributor to a vacuum advance one, and put an aftermarket electronic ignition system in it, as the points had to be changed out every month which was downtime and money. I also got rid of the A.I.R. pump system, since it was not required on heavy duty trucks, and all the dust ate those things for breakfast, costing more money. Getting to the back spark plugs was a nightmare, so I have always used Neversieze every time I change plugs, no matter what vehicle I am working on.

Thing is that a Suburban is a pretty heavy light vehicle, so what kind of gas mileage were you expecting, with a 6.0 liter H-P gasoline motor? What does it do now, both city and highway?

 

For what it’s worth , I have a 2008 Suburban with a 6 liter, 6 speed transmission and 3.73 rear end that gets 19 mpg on the highway.

SD

 

Thats about what I would expect from a Suburban.

 

Hell, I'd be ecstatic if I got 19 highway on my '08 2500 Burb. Best I can do is about 15, and at my usual highway cruising speeds (80-90) I get like 12-13. At least I have a 39-gallon tank so I don't have to stop every two hours. On my 26-gallon Denali, I'd swear I can see the gas gauge dip every time I nail the throttle.

 

My 19 mpg is at 60-70 mph.

SD