You'll save time and energy by checking the ignition timing because this test will additionally tell you whether the mechanical timing is off. Kill two birds with one stone.

I always check my plugs for proper gap.
I also always used spark-plug antiseize when installing new plugs.

I suspect the engine timing is off as well...or your math? :)
That is a huge drop in mpg's and I would think your ignition timing is too far advanced.
What is the spec anyways, 18deg. ATDC?

O.K. So it has been a week or two since my original post. My SECOND tank of gas ran to 380 miles; an increase of 100 miles.

I checked my spark plugs this weekend, and set them all to .041. The specs are .039 to .043 so, they are all smack in the middle. Only one was off and it was set a hair to large (.045 or so). I believe the 3rd tank is half way done so, I will see how it goes and keep you all posted. The timing gun seems like a great idea but I would rather save the cash if everything is where it needs to be.

hi... were you able to fix ur issue .. cos i also have the same after changing my timing belt...etc

Sounds like your mechanical timing is off.

My timing belt is/was NOT off. The first tank I only got like 280 miles out of. Since then, it went right back up to 380-400.

Not sure why the first was so low, but it is back to normal now. I did check the spark plug gap (as was recommended) and they were all about good. I am not sure what to tell you besides give it a thousand miles or so to warm up!

good advice is to ALWAYS check your spark plug gap before installing new plugs. just because they say pre-gapped doesnt mean anything. take for instance the guy at the parts house who drops the box of plugs down a little hard on the bottom. pre-gapped plug just gapped itself a little smaller...

Spark plugs come packaged with a pretty damn stiff piece of cardboard protecting the tip...

Either way though, you should check them.

regardless used to have a gf who worked at o'reillys and to prove that pre-gapped plugs arent always pre-gapped they pulled out a box and checked the gap. about every 4th plug was off. im just saying its always good habit to check them.

I agree that it's good to check them first. I was just saying that dropping the box would probably not damage the plugs or mess with the gap because the tip is protected.