Question about tires.

[Csilvy2000]

You may want to take a good look at that gas tank. Sounds like you may have a hole in it.

 

[Wraith]

When's the last time you did plugs, wires, cap, rotor, timing? That would have more effect than the tires and 8-9mpg is a whole other batch of problems that's not good to start that don't include tires unless all four are different sizes and running 5psi.

 

[Wraith]

You may want to take a good look at that gas tank. Sounds like you may have a hole in it.

That's an interesting thought, are the neighbor's siphoning your gas?

 

[grandamman]

I have an '85 Monte with the stock 14" steelies and the 305. When I bought the car I averaged about 8-9 MPG with the set of 1987 snow tires (that still held air!) that it came with. I had to replace them with newer all season tires to pass inspection. These tires are significantly smaller in diameter than the snow tires, and at least one of them has a noticeable weep. Around that time my MPG dropped to an apalling 6, even after a carb tune-up to lean things out a bit. My question is this, is the smaller diameter the main reason behind the loss in economy, or is it the loss of tire pressure? (I pump the leaky one up every few days, it seems to have slowed over time). I can't get the leak fixed by any big box store, the shop that fitted them sold me old tires and no RMA certified place will touch them. Could it be that I need an alignment?

Yes. Smaller diameter tires will negatively effect fuel economy.
I would imagine you are running higher revs too

 

[Ribbedroof]

Lots of good input

Makes the 13.5 mpg the 267 in my wagon gets look absolutely stellar!

 

[UNGN]

When's the last time you did plugs, wires, cap, rotor, timing? That would have more effect than the tires and 8-9mpg is a whole other batch of problems that's not good to start that don't include tires unless all four are different sizes and running 5psi.

This is good advice. Also Check the transmission fluid to see if there is any,

What RPM are you running during these 6 mpg drives? 2,500? 4,500?

How much vacuum does the car make at idle and going down the road? A car that gets 6 mpg just driving around should smell like unburnt gas.

Does the car smell like unburnt gas?

 

[Bludacious]

Gas tank, cap, rotor, and plugs are all new, transmission fluid is in good shape and also changed the oil. Timing has been checked too. I also painstakingly went through and redid the vacuum routing to eliminate all the leaks. I'd say on average while cruising the revs are around 2000 if that, I have the clock in my cluster in place of a tach so I'm not 100% sure. Since I posted this I've rooted out the main culprit as the carb anyways seeing as it almost constantly has fuel residue on the outside and though I've gotten used to it my passengers notice a reek of gas. I've leaned it out as much as I can without getting serious stumble at idle and currently can't afford the time or cash for a rebuild as it is my daily so for now my strategy is drive like a grandma, seems to work a little bit..