Nice looking truck.
I have a '79 super cab that had a 460 and 4 speed when I got it. On a good day I could get in the neighborhood of 10 miles per gallon. On a bad day it was about the same. I had a '79 F150 with mild built 400 and automatic that got about the same 10 MPG. When I first got that truck I thought it was getting about 18 MPG, but later found it was drawing from both tanks while I thought I was only using the front tank. When the back tank went dry I was out of gas even though the front tank was 3/4 full. Are you sure your initial trip was only drawing fuel from one tank? While 7.5 MPG sounds a bit low, 12MPG (at least from my experience) seems generous.

 

Does seem low. My 2wd 400 with 4.10 rear and 29" tall tires will return 12.5mpg screaming down the highway at like 3,000 rpm. My former '78 4x4 with 3.50s a 400 and 33" tires, lifted, would get 14mpg on the highway.

What fuel are you using? Ethanol 10% blend will cost you 2 mpg in my experience. Also did you degree the cam to be sure it was installed straight up? I wouldn't sweat 22" idle vacuum. If you are at low altitude that seems reasonable.

plug seems just 1 shade dark, maybe try going one step leaner on your edelbrock jet/rod chart.

 

Thanks!

I never had any indication it was drawing from both tanks but I have the front tank bypassed just in case and have had it that way for a while.

I would be ok with 10mpg and while that's only 2.5 more than I'm getting now it's still would be 25% better

 

For sure it's 10% Ethanol, the only places that sell pure gas are pretty far but I'll fill up with it and see if I can.

I'll try one step leaner this weekend

 

Filled the truck up tonight and got 8.5 mpg... better!

Changes so far for this tank are, 98 jets with 71/57 rods. 6" hg step up springs (pink) and disconnected the secondaries.

The pvc valve was plugged and I fixed that yesterday so we will see if that helps any on this tank.

Per hivoltj I'll swap to one step leaner on the jet/rod combo tomorrow and see how it runs. 3 steps leaner ran like crap so I know that's too far.

Thanks everyone for the help so far!

If next tank isn't any better I may try swapping in the old skinny 235/85r16 tires and wheels for a tank just to see what happens.

 

Good work on figuring it out... gotta say there is always some good advice here.

My 2c worth would be to rethink the C6. They are huge, heavy, antiquated bits of equipment that cost a lot in both power and mileage. No overdrive is just salt in the wound.

I'd look into a conversion to an overdrive trans, or even a manual conversion if you really are chasing economy. Automatics are always thirstier than an equivalent manual due to the fluid pump having to be driven - it can take much as 30% of a vehicles drive power at a sustained speed.

Sounds like you're close with the carburettor, keep up the tuning and you'll hit the sweet spot.

On the tyres, 235/85R16 or 7.5R16 are great for economy when compared to aftermarket 33 or 35" tyres. I like my Bridgestone Desert Duellers in 7.50R16.

- boingk

 

I love that size tire. Have them on mine. Not a lot of choice though.

 

Future plans do include an E4OD and fuel injection but for now this is my DD so it can't go under the knife for that much work.


I am looking into 275 or 285 width tires for 18" wheels that could replace the 325/60R18's I have now. The current tires (Fuzion) suck in the snow. Too bad the current tires only have 4000 miles on them.


I did the the O2 bung welded into the passenger side header this weekend so now I just have to wire in the wideband O2 sensor and I'll be able to get a good look at where I currently stand. I'll get the driver side welded in some day soon but the O2 meter only has 1 02 sensor so no need for now

 

Wish I could get those type of numbers. 400 35in tires and 4.10 gearing is closer to 4mpg than to 14 mpg haha

 

Have you given any thought to retarding the cam? I've got a feeling that your dynamic compression ratio is on the high side.

"The motor is a 400 with badger flat top pistons (still only 8:1 compression), edelbrock 2172 cam which is 204/214 at .050. "

I punched these numbers into a compression ratio calculator. 4.03 bore, 4.0 stroke, .040 head gasket thickness, 78cc combustion chamber, 2cc of piston dish for the vale reliefs, and .016 below deck piston height. I came up with a tic over 10 to one compression.http://www.csgnetwork.com/compcalc.html

I just cant see how to get to 8:1 compression with flat top pistons and stock heads which have chambers between 74.5 and 78.4 cc volume. http://grantorinosport.org/BubbaF250/parts/parts02.html

 

Very nice truck !!

When you were running on 87 octane, before the rich fuel and timing adjustments, was your engine pinging ?

When running on 91 octane, does the engine ping at WOT ?

You can ignore Edelbrock's 14'' vacuum statement !!!

22'' at 750 rpm at sea level is very good, maybe too good.

Is that with vacuum advance connected to manifold vacuum ?

Compressions of 170-175 in all 8 cylinders is very healthy.

That suggests a Static Compression ratio of +/- 10 as indicated by palmrose2.

Did you hold the throttle fully open when checking those ?

I'm glad you replaced the PCV valve before it caused gasket failures and oil leaks !!
Ensure that the breather on the opposite valve cover to the pcv valve is not clogged and that the air filter element is clean.

The O2 sensor and wideband AFR gauge will assist with the fueling, but not the ignition timing.

Plug the vacuum advance and read the timing at 500rpm intervals starting at 1,000rpm up to the 32 degree limit. (Then reconnect the vacuum advance.)

Put the numbers onto a graph and the line should be pretty linear. The 32 degree limit could be met at between 2,500 rpm and 3,500 rpm.

A faulty dizzy that doesn't advance properly will steal gas mileage big time !!

 

Static or dynamic compression ratio? It matters...

4:10 gears is certainly part of the problem. You're running high rpm on the highway with that short of a gear. Something taller like going back to 3.5's would help.

Static compression ratio or dynamic compression ratio?

Static compression ratio is about as useful as a concrete parachute...

Dynamic compression ratio... now that's where it's at! That truly tells you how the motor is breathing (don't forget it's an air moving device).

 

On the contrary, the static compression ratio is incredibly useful, and assists in designing an engine upgrade/improvement.

Without it, choosing a cam which obtains an acceptable DCR ratio, would be very difficult.

Not knowing the exact deck clearance and combustion chamber cc's on this engine renders calculations almost useless.

It may well be an inefficient engine build with regards to the squish !!

 

I got these, about $300 cheaper for 4 than anything I could get locally

https://www.ebay.com/itm/Mickey-Thom....c100290.m3507

 

Both are good but you can't always trust the cranking test either. Once I had a 302 in my shop with a little vibration/misfire. Cranking compression showed all cylinders withing factory specs but the cylinder balance showed one cylinder not doing it's job very well so I left the compression tester in the weak hole and started the engine, released the compression from cranking and with the engine running it could only build 55 lb of pressure. Sadly the customer didn't let me go any deeper, just traded the vehicle off.