Tbi 454 mpg...Is this as good as it gets?

[whiteboyslo]

So, I bought my 1993 C2500 Suburban last October and have been going through the whole truck. It has 147K miles and runs great, but the mileage STINKS! Today's fillup was best so far, but still just under 11 mpg. This was on a tank where the driving was predominately freeway at 65-70. I drive it like an old man, especially around town.

Everything done so far that could/should help MPG...

• Spark plugs (ACDelco Rapidfire)
• cap, rotor, wires, coil (all ACDelco)
• All filters (including AFe air filter)
• All fluids
• Transmission serviced and checked out. Clean bill of health
• Brakes serviced (no dragging or binding)
• New oxygen sensor (ACDelco)
• New cat (CA, so can't remove)
• New Magnaflow straight-thru muffler
• New PCV valve
• Added the stock chin spoiler. Picked up 1MPG on the next tank, so this seemed to help a lot
• Steady diet of Seafoam in the tank. Injector spray pattern looks strong.

Possibly not helping....

• 265/75/16 tires. It's a highway tire, so tread is not aggressive at all but it is wider. Speedo reads very accurate per all the speed traps in my area.*edit* Just did the math. And adjusting for the tire diamater difference, MPG on the last tank was 11.1. Better, but still not great.
• Might be my paranoia, but I THINK my torque converter might not be locking up at higher speeds. Now that I have an exhaust loud enough to actually hear the truck (no tach), it sounds like the revs never drop once I get to cruising speed and don't jump up a bit when I roll into it once cruising like I think they should. Any way to check/confirm without having tach?

The truck is running fantastically otherwise and definitely seems to have taken a liking to the minor exhaust work, but this mileage is brutal. Fuelly.com has me believing that I should be seeing low teens when it's mostly freeway driving. FWIW, the PO insisted he got 16mpg on a trip from CA to ID, but who knows how true that is.

Ideas?

Mike

 

[biggbiker]

11 MPG is what my 1999 Yukon 5.7L gets, driven by an old man around town. 16 is the best managed on highway.

 

[c1500deathtrap]

other than verifying that your CTS is functioning as it should thats all i can think of to help. maybe change out the rear diff fluid?

 

[whiteboyslo]

other than verifying that your CTS is functioning as it should thats all i can think of to help. maybe change out the rear diff fluid?

CTS?

Diff fluid was changed, too. I used the ACDelco stuff, the type without the LSD additive per the TSB that affects my truck.

Mike

 

[Supercharged111]

11mpg is AOK for that rig. My dually gets 10 on the highway. They're just pigs, no 2 ways about it.

 

[someotherguy]

Yeah you're pushing a super heavy truck around on big heavy tires, heavy wheels, turning over a 14 bolt 9.5" rear (or 10.5" depending on model), with a whole lotta cubes under the hood.

Best you can do is be sure it's in top state of tune, avoid any gimmick MPG/power "tricks", tires inflated to proper pressure, and try to keep your foot out of it.

Or trade it for something else..

Richard

 

[magimerlin]

Your driving a heavy vehicle with the aerodynamics of a brick...mpg ain't gonna happen, but gpm(gallons per mile) might though..lol.. not to sound rude but..you want great mileage you gonna have to buy a honda..lol.. typical mpg for the whole gmt400 line I've seen reading posts is as bad as 9 and upwards of 20 or so depending on driving conditions, vehicle conditions and any added options(weight), altitude, fuel choice, ambiant air temp, city driving%, highway driving%, tire air pressure, how heavy your foot is on tje go pedal, etc. You get the idea.... There is alot that contributes to fuel mileage even if the motor is running 100%. Just food for thought.

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[whiteboyslo]

To clarify, I knew full well going into this beast that MPG is not its strong side. A Prius it isnt (though it sounded like one until i got the exhaust work).

My bigger concern is that the poor MPG is indicative of something else being wrong. And yeah, if the problem is a cheap part that can pay for itself in a tank or two, I'd be interested in getting it sorted.

Mike

 

[AlexHawk94]

On my 5.7 vortec I have approx 5-8 gallons per 60 miles

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[Supercharged111]

If there is something wrong it's not affecting your mileage. That's what they get on a good day.