Terrible gas mileage,
 

I have been tracking the MPG on my truck since i have had it, it has finally settled on about 6.2 MPG, OUCH!!!!

I have a 500 CFM Holley on it, recent tune, valve adjustment, runs like a top just horrid mileage. I have the stock 3.54 gearing and a 420 tranny. The entire fuel system was replaced after I bought it

I did try jetting it smaller than what came with carb, mileage went down to 6.1, so I put the stock jets back in, went up to 6.27. I can go smaller as I bought 2 sets, each going down one notch from stock. I do have a big air cleaner with a paper filter so it is getting a lot of air.

the elevation here is 4450

Re: Terrible gas mileage,
 

Lots of things can affect mileage. Some things to check are ignition timing, tire pressure, brakes dragging, and front end alignment. I'm sure others here can suggest more to check. Are you 100% sure the rear end is a 3.54? You should be getting closer to 10 MPG if not better. If it sits for a long time, you could be losing some fuel to evaporation. Lots of hilly driving obviously affects mileage too.

Re: Terrible gas mileage,
 

As said, we don't know your usage, loading, terrain and possible Holley carb internals. Even with a 305, that's pretty low MPG. Unknown history from PO. If you 100% positive about ratio, great, but could have 3.92 real easily. You can't do accurate ratio calculation by tire/shaft rotation.
You may be falling into the category of "playing with it too much", "modifying" too much. Original factory motor Synergy gets out of whack. Go back and try a 350 CFM original carb. You could be wasting gas.

Re: Terrible gas mileage,
 

I am pretty sure I have a 3.54 rear end, I found a tag on the 3rd member cover indicating the ratio and that it was a posi rear end. All the hydraulics have been rebuilt, the brakes don't feel that they are dragging when I put the truck in neutral and coast to a stop light.
I do have the tuning book for the 2300 series Holley, I remember reading that if you lean a mixture too much that more fuel will be used to do the same work. I have a Summit warehouse in town so getting jets a just a short ride away, maybe I am going in the wrong direction with jets.
Since tomorrow is gonna be nice, before another storm, I will check the timing and pull a plug to get an idea on how they are burning.
As far as driving, it gets used just around town, going to dump transfer station, errands, and no major driving in the Sierras.
Fuel could be evaporating or my speedo could be off, too, like Ed said. I did notice that the needle is jumping a bit, indicating that cable needs lubrication. I still have the original carburetor and air cleaner, so I could put it back on, but it sure runs better with the Holley, the only thing I had to reroute was the choke cable since it comes in on the right side on the Holley.
Today when I was driving around I smelled gas, found a leak on the fuel line going to carburetor, fixed that so that may have contributed to poor mileage too, but it hasn't been driven much in the last 6 months, that only would leak when the motor was running anyway.

Re: Terrible gas mileage,
 

jagarra, what size tires are on the truck? If you have 15" tires, your engine will sound like a screaming dive bomber. That will kill fuel mileage. Also, what type of gas are you using? If you are using 87 octane gas, you are burning 10% corn squeezings and that will affect fuel mileage as well. Driving habits also have a great effect on fuel mileage, too.

Re: Terrible gas mileage,
 

Lewis,

I am running 15" tires, 7.00x15 so the OD is about 30". I run regular, I have been using pure gas, no corn when I can get it, the last 2 tanks have been this fuel.


A bit more information. I have been running this carburetor since May of 2015, problem is that I never dated my mileage figures. I believe I got it running and licensed it around January of that year, so the mileage has been steadily declining over the past few years.

Re: Terrible gas mileage,
 

I would check the float level. It might be too high causing it to be rich. Remove the plug on the side of the carb. With the engine running it should barely run out.

My truck is still running the stock carburetor and last weekend it got 15.1 mpg (been averaging 14.5 mpg). I have a 3.54 rear axle and 16" rims with LTX M/S2 LT235/85R16E tires.

Re: Terrible gas mileage,
 

I am going to check a lot of stuff today. Ran some figures to see how things were changing.

original carb 337.3 miles used 41.4 gal average over that period 9.11 MPG
Best mileage 11.18, worse was 7.7 MPG

Holley 844 miles used 102.2 gals for an average of 8.25 MPG
Best was 11.4 on the first tank with the Holley
worse was 6.1 MPG as of the fill before the one yesterday.

looks like i have a lot of items to check to see what has changed.

One thing that pops out is the truck doesn't like pure gas, will see how is goes on this tank. Does pure gas evaporate faster? Truck sat unused for quite awhile before the bad MPG figures.

Re: Terrible gas mileage,
 

Gas can go bad in just a couple of months. With our truck, the tank is vented at the cap. This allow the tank to breath the outside air. This air has moisture and oxygen in it. The moisture will condense into water and lay in the bottom creating rust (or mixed with the alcohol in the fuel). Then the oxygen will blend with the fuel making it darker and smelling foul. All of with it will make the engine run poorly (poor mileage too). If the fuel cap was like the new truck the entire fuel system would last for several months without problems.

What I would do if the truck is running badly, topped off the tank the drive it until it is low on fuel then topped it off again and the problem will go away (providing nothing else is wrong with the engine).

Re: Terrible gas mileage,
 

Well I think I have solved part of the problem. I seem to go through the plugs on the opening that are used on the carburetor to block off vacuum ports, this is the 2nd time I have had to replace some of them. I found cap that had disintegrated enough to allow air to come into the carb base. Checked plugs and they were a little rich, still brown but not tan. Dwell was right on, also checked vacuum at idle, it was 12.

Went for a drive including a trek up the mountain to Virginia City, down into the valley and to home, filled up again, got 11.19 MPG. :upyes:

Re: Terrible gas mileage,
 

Glad to see your mileage improve. After a super-tune on my '66 305, I got 15 1/2 Hi-way.

Even though many like Holley's I had no problems with my original. I would always adjust it to the Highest Vacuum that I could get out of it. Also what kind of Muffler are you running? V-6's love to breathe. I always ran a Straight-Thru on mine.

Re: Terrible gas mileage,
 

I'm using a Flowmaster 942040.

Re: Terrible gas mileage,
 

Mine has the stock exhaust system, 2 into one with a muffler.

Re: Terrible gas mileage,
 

Is the Muffler the original GMC V-6 type? There is a chance that it won't be because they have been dis-continued for many years now. I wouldn't run anything less than a Straight-Thru. A Cherry Bomb now made by Thrush is a good replacement. As mentioned earlier it's very important for these engines to breathe and to get the Mileage that they should be achieving.

Re: Terrible gas mileage,
 

It could be the original muffler. The truck had 67K on it when I got it. When I opened up the top end to check out why it didn't run, the heads and inside the valve covers were really clean. I found 3 exhaust push rods laying loose. I pulled apart the the intake to check the lifters. A friend of mine got it as a non runner about 6 years prior from a bee keeper friend.
Dual exhaust would be nice, haven't priced what that would cost. Not sure if I would like noisy exhaust though, rather have something quieter.

Re: Terrible gas mileage,
 

For mufflers there are other straight through designs like Magnaflow, they say the longer the muffler the quieter with this design.:upyes:

Re: Terrible gas mileage,
 

Since we are talking about mileage, what are the correct plugs for a 305D motor? When I go to Rock Auto I see either 3/8" reach or 3/4" reach. My Motors manual says C44S for a plug, but it just list the E motor. I remember there was a reach change somewhere in that era, but am unsure which motor takes what reach.

Re: Terrible gas mileage,
 

These trucks were a little loud from the factory, but as long as you run the Exhaust all the way to the back with a factory type or custom Tail Pipe, it shouldn't be too noisy for you.

If you don't like noise, you would not have liked my '62 6500 with the 478 V-6. It had factory Duals, factory Straight-Thru Mufflers and they ended underneath just a little passed the cab. Especially so with a full load going uphill, lol.

We would carry 20,000 routinely.

Re: Terrible gas mileage,
 

Jagarra, I would pull a plug and see what length it is and replace accordingly.
You wouldn't want to replace a short reach plug with a long reach due to the carbon build up on the lower threads.

Re: Terrible gas mileage,
 

There is two styles of heads you can have. Looking at the sparkplug hole if you see threads at the top of the hole it is short reach plug. If it is down lower into the head it a long reach plug. Beware it is possible to have both style on an engine. I have an engine that has both style of heads.
:banghead:

Re: Terrible gas mileage,
 

I gonna pull a plug on each side and stick a spring hook down into the hole to see what's up. I have a set of short plugs in there now, but I found 2 boxes of the correct NGK plugs in my stash that are long reach.
When I pulled them the other day it seemed that they didn't screw down very far until they tightened up. Short plug in a long hole?

Re: Terrible gas mileage,
 

Here is what the different plug holes look like.

Re: Terrible gas mileage,
 

I have a 62 Owner's manual, it list the C44S plugs for the 305D, This number as old as it is most likely has been superseded with a different number. I would say you have the 3/8 reach plugs.

Re: Terrible gas mileage,
 

I also have the original owners manual in my glove box, just didn't think of checking it. The C44S was the plug listed in the Motors repair manual for the E model so looks we are narrowing it down.

During my searching ran across a great site for cross referencing sparks plugs, has cross to everyone and explains what all the numbers and letters mean.

http://www.sparkplugdepot.com/cross

thanks guys,

gg

Re: Terrible gas mileage,
 

Gents, a D engine would have ended production in '62. The .375"/.750" plug length problem ended in '63 with the appearance of the E engine. The picture you posted jagarra is a set of heads with the .750" reach plugs. The original AC plug for that engine was the 44NS non-resister plug. AC superseded all N style plugs with the XLS style plug. I picked up 2 sets of C44NS plugs on evil-bay several years ago. I like the "C" plugs because of the thicker electrodes these plugs have. The C plugs were originally the "commercial" version of the plugs the light duty engines use. The reach controversy arose because early engines were burning up plugs and cracking the heads around the plug port due to casting thinness. The .750" reach heads is the correction of this problem.