4.1 mpg?

[84prspctr]

I know the fuel mileage shouldn’t be that bad. Every time I take the truck out I’m stopping at the gas station at least twice, which isn’t making the wallet happy with gas being over 5 bucks a gallon. Timing is set to 12 degrees initial, carburetor is fine tuned with vacuum gauge, idle mixture screws are about 1.5 turns out. Accelerator pump on second notch. Edelbrock 1406, hei distributor with accel super coil, none of the valves seemed loose or out of adjustment. Holds timing steady no jumping or anything. 35” tires lifted 6 or 7 inches, turbo 400, 350 engine, Holley contender intake, stock mechanical fuel pump, no emissions, hooker headers. I’m running about 1/3 throttle going 65 on the highway. Is there a way to tell if my vacuum advance is messed up?

 

[mxer147]

Are you certain your odometer is correct? Do a mileage check on the road to confirm accuracy. I didn’t recalibrate my Duramax after a larger tire change so the miles and speed was incorrect.

 

[84prspctr]

I don’t have a working speedometer or odometer. I track it through a gps speedo

 

[Rickf]

With something like a turkey baster and the cap & rotor off you can pull a vacuum and see if the advance plate moves and holds.
Any idea what is your axle ratio?

 

[84prspctr]

Not completely sure on the axle ratio. It’s turning 3000 rpm at 70, about 2500 at around 60 or 65

 

[85K304SPD]

Small engine, big tires... sounds believable on the 4.1 mpg. Math doesn't lie.

 

[Bextreme04]

I'm in a 1980 K25 that weighs 5400lbs with me in it. With 35x12.5" tires, 4.10 gears, 350/TH400/NP205, and 4" lift. I'm running the stock mechanical pump and quadrajet tuned with an AFR gauge and even when it wasn't dialed in I just drove 160 miles round trip on the freeway going to the annual squarebody group BBQ. I filled up, hit the road, and filled up again after getting back. I averaged 11.5 mpg running at 65-70mph on the freeway. I probably get about 10-12mpg around town... but haven't officially checked it in a long time. 4mpg in a 350 means you are pig rich most of the time. Pull a spark plug and see what it looks like. I bet it is all sooted up.

At that kind of mileage and if you drive it regularly, an AFR gauge and edelbrock tuneup kit for the carb would pay for itself pretty quick.

 

[SquareRoot]

I'm only getting around 15 mpg at 65 mph.

 

[Bextreme04]

I'm only getting around 15 mpg at 65 mph.

Fuel injection and overdrive are wonderful things!

 

[Matt69olds]

How are you calculating distance driven?

Fill up the gas tank, drive a known distance. Use the mile markers on the highway if needed. Stop and refill. Divide the miles driven by the number of gallons needed to refill the tank.

Unless it’s way out of tune, or leaking fuel somewhere, I find it difficult to believe a sensibly driven well tuned small block truck can only get 4mph.

 

[squaredeal91]

My Grandpa had a 68 f250 w/360 and auto. 2 barrel Running on 7 cylinders and smoke rings for years. No gas gauge drove it with a mileage log and never over 55 for fear of explosion unless it was downhill with a tail wind and it still got 7mpg lol.
can you tell if you're in the secondarys at 65-70? That would explain some. And how big are the jets. Maybe it's overcarbureted? In many cases I've known edelbrock carbs to be over jetted out of the box.

 

[84prspctr]

I'm in a 1980 K25 that weighs 5400lbs with me in it. With 35x12.5" tires, 4.10 gears, 350/TH400/NP205, and 4" lift. I'm running the stock mechanical pump and quadrajet tuned with an AFR gauge and even when it wasn't dialed in I just drove 160 miles round trip on the freeway going to the annual squarebody group BBQ. I filled up, hit the road, and filled up again after getting back. I averaged 11.5 mpg running at 65-70mph on the freeway. I probably get about 10-12mpg around town... but haven't officially checked it in a long time. 4mpg in a 350 means you are pig rich most of the time. Pull a spark plug and see what it looks like. I bet it is all sooted up.

At that kind of mileage and if you drive it regularly, an AFR gauge and edelbrock tuneup kit for the carb would pay for itself pretty quick.

The carburetor is a couple months old. I pulled the plugs a couple weeks ago and found it to be a little on the lean side, a tan ish color.

 

[Bextreme04]

The 350 that was in my truck before had some small valve 305 heads and a Comp 270h cam in it. It would detonate with any kind of decent timing in it and ran terrible. I had at least one piston with the compression ring shattered into a million pieces and most of the other cylinders had leaky valves. It ran pig rich too and STILL was getting 7-8 mpg.

 

[Bextreme04]

The carburetor is a couple months old. I pulled the plugs a couple weeks ago and found it to be a little on the lean side, a tan ish color.

Then there is ZERO chance that you could be actually getting 4mpg unless that motor is pumping out 800hp and you are driving it floored everywhere you go.

 

[SquareRoot]

Plug color is meaningless with pump gas. My efi runs at 12.5 AFR @ WOT, 13.5 @ idle and 15+ at cruise and they look the same as the when I took them outta the box 10,000 miles ago.