I'm out of ideas so it's time to turn to y'all for help...

Here's what I have, '79 F350 supercab 4x4. C6, 4.10 gears, 33" tires.

The motor is a 400 with badger flat top pistons (still only 8:1 compression), edelbrock 2172 cam which is 204/214 at .050. Edelbrock says 14" of vac at 1000 RPM but I'm getting 22" at 750 rpm. Edelbrock intake and an edelbrock carb. Carb is jetted with 98 primary and 95 secondary with 71/55 rods.

I'm running 91 octane, to make it run on 87 it had to have no vac advance and run super rich so I gave up on regular gas.

Now here's my problem... I'm getting 7.5 mpg with mostly freeway driving at 60 mph. I tried the 98 jets with 76/57 rods but got the same mileage and the truck didn't run very well (too lean). I also had 101 jets with the 76/57 rods and it ran well but same mileage (this is mathematically the same for cruise as what's currently in there).

For now I have disconnected the secondary's so I can eliminate that part of tuning.

Timing wise I'm at 16 deg at idle and 32 deg max with 45 deg max with vac advance.

Spark plugs look gray/brown-ish

Does anyone have any suggestions as to where to go from here?

Random other things I have checked, brakes not dragging, trans not slipping, floats are adjusted correctly per edelbrock. I don't know the fuel pressure but the level in the float bowls is correct every time I take the carb apart so I don't believe I have an over-pressure situation.

Thanks in advance for any help

Here is the truck in question

 

Those quad headlights are cool!

Actually for the truck and how it's equipped I'd say you're in the expected range of fuel economy, perhaps on the lower end.

A 400 in an F100/150 dent is going to give you max highway economy at no more than 15-16 MPG. Going downhill with the wind behind you. That's with narrow tires and a manual transmission and 2wd.

An F350 SC 4WD with low gear ratio of 4.10 or 4.56, a C6 automatic, wide tires... You'd be lucky to get over 10.

If you've noticed a recent and sharp decline in mileage, let us know and we'll try to help troubleshoot. Ignition timing or spark issues comes to mind as an area to look at, if so.

 

Here is my issue... when I bought the truck and drove it home 300 miles it got 12 mpg. Granted that was with stock tires, stock motor and stock carb but still had the same 4.10 gears and same trans.

The new motor with strait up timing, headers, mild cam, and dual 2.5" exhaust should be more efficient than the stock motor. Wide tires will hurt but the drop in freeway RPM from 31ish to 33ish tires should help some too.

That said I don't expect good mileage at all but going from 12 to 7.5 is a 40% drop and that is huge.

 

When did you notice the decline in fuel economy? Also, was it sudden or gradual? Any odd exhaust smells or changes in appearance? Any 4WD driveline wear or changes? An often-underappreciated issue is the additional wear that wide tires often impart to the entire drivetrain. Misalignment of suspension and/or driveline wear can dramatically reduce fuel economy as much or more than engine issues can.

By way of example, my son recently purchased a '75 F250 2WD 390/NP435, a very nice truck that was getting terrible mileage and running the owner nuts, so my son got it at a crazy low price. Just a carb rebuild, timing reset, new front suspension correctly set, new carrier bearing, and new u-joints took the highway mileage from 6 to 14 MPG. Right where it should be. So, tuning, alignment, drivetrain, tires... check them out carefully. In general, any engine or other modifications from factory stock should be double-checked for complicity in driving down the mileage.

 

Truck got 12mpg on one road trip and then spent 5 years being restored. It's been getting 7.5 since being back on the road. So it was an immediate drop in mileage.

All u-joint have been replaced, axle bearing check and front wheel bearings were replaced. The only thing I haven't done is check front axle toe in. It drives very well but I will check that today and if I can't get a good reading I'll take it in for a toe alignment.

I still have the old tires and wheels which were actually almost new when I bought the truck. If I can't make any improvements I'll run a tank of gas with those on to see if the 325/60r18 are some of the problem

 

Does anyone with a rebuilt 400 know their jet and metering rod settings of their edelbrock carb?

I'd like to know/confirm that I'm in the right ballpark with mine.

 

Just for good measure I ran a compression check today with the motor warm, got 170-175 in all 8 cylinders.

 

How are you calculating your mileage? You've got an 8 or so % change in your rolling radius with your new tires.

 

All the gauges are aftermarket and the truck was a 3.50 geared truck from the factory (original owner put in the 4:10's)

Luckily for me with the new tires all that added up to a speedo that, per gps on my phone, says it's 1-2 mph slow at 60mph. So 3ish% off.

I have factored that into my mileage.

 

That is high for 8:1 deal.

So all plugs look good and the same, engine seems to run as expected power wise, and you checked for wheel drag after a 25 mile or so trip?

I agree 7.5 at 60 is not right.

 

Did you change the cam? While not much gain in circumference going from 31 to 33, the width can cut it down quite a bit, expect 1-2 mpg minimum. Seems like a lot of timing to me, did you try backing that down?

 

Here are what the plugs look like from today



I agree that 175 does seem high. I'll check it again when the motor cools down. I did the math on the 8:1 when I had the heads off but it's possible I did it wrong. I did not cc the heads so if they were cut at some point that would explain it. 8:1 with the cam that's in there should support regular 87 gas and this motor won't run well on that. Engine runs well, idles smooth at 800 in park and 650 in drive and over has plenty of power.

I did just find that my pvc valve and breather were in bad shape so that has now been replaced

On a side note. Factory tires for this truck are 7.5-16 which are 31.4 tall. The standard replacement for theses are 235/85r16 which is what this truck had when I bought it. Width contributes to rolling resistance so the new 325/60r18 are for sure wider but I just weighed them and the old tires today. The 325's with aluminum 18x8.5 wheels weighed 90.1 lb and the old 235's on factory steel wheels weighed 80.2 lb. I was supprized that the new bigger tires are only 10lb a tire/wheel heavier.

 

Sustained diving speed vehicle and wheel and tire wt are small players....but good data nonetheless.

For reference, my 10.5:1 with comp magnum 230 was 180 psi....however more compression more efficient....so not your problem.

Even though the plugs look good it seems like the next logical step is to put a wideband on it and start driving.

 

The reason I asked about the cam is because cam profile will affect cylinder pressure as much as static compression ratio.
One time I went from 7.50x16 to 31x10.50 and lost about 1 1/2 mpg so fatter tires make a lot of difference over pizza cutters. I'm kind of a fan of tall narrow, old school, tires now but the choices are few and generally expensive. Next week I'm getting 255x85x16 for my 59 because I'm tired of looking at the wimpy 235 street treads.

 

Yup that's where I'm at too. Hoping to find time to weld the O2 bungs in this weekend.