Terrible gas mileage,

[BillT]

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.

[James]

I'm using a Flowmaster 942040.

[jagarra]

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

[BillT]

Originally Posted by jagarra

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

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.

[jagarra]

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.

[bigblockv6]

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

[jagarra]

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.

[BillT]

Originally Posted by jagarra

Not sure if I would like noisy exhaust though, rather have something quieter.

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.

[snazzypig]

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.

[James]

Originally Posted by jagarra

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.

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.