I do believe you have a bad ground connection.    A bad ground makes the fuel gauge read much higher than normal, and can make gauges read poorly.

Un plug the fuel sender wire at the tank and see if there is a short to ground in the wire going to the front. if it goes up you have a short in the wire, as above also check your ground tank strap/wire, and check that the sending unit has a good ground not just the tank.

I have to agree with everyone else. The problem most likely occurred due to the amount of time the light was on, draining the battery. It prolly drew enough amperage on the ground(s) to create a faulty connection, or finish off one that was iffy to start with.
The speedo problem is going to be a totally different issue, even tho it started at the same time. The only way it could be related would be if the speedo cable became a new ground for the gauges after the bad ground gave up the ghost....highly unlikely, but...
Start checking those grounds...

If you had an extra battery, you could swap it out just to see what happens.  Sometimes they act funny when drained...

Since you fiddled with the battery.... check the ground wire from the battery to the frame 1st.

Did you mess with any other areas recently as well?

I know mine is the side post connection.  I can tell you from direct experience (on my GMC truck) that the connectors in the side of the battery can fail and you won't know it.  My truck started and ran just great, but every once in a while when you shut it off, the lights went dim, flashed, stereo came on and went off and locks would lock and unlock.

The ground side had snapped inside the battery.  Contact would generally be there but not always.  I finally found it one cold nite when I had just come in, shut it off, it did it's wierd thing and I had to go back out for something.  She wouldn't start so I lifted the hood and checked the cables on the battery.  when I touched the ground, the light came on under the hood and inside (I had left the door open).

I moved the ground and you could hear it making and breaking contact, kind of a crackly sound.

I replaced it and 500 miles later, the same thing happened.  I then tried the screw in studs, but it happened a third time. 

I changed over to a top post setup and have never had another problem with the battery.

Thanks guys. I will check out the grounds this weekend. I didn't disconnect anything when I charged the battery. I have an aircraft type connection mounted to charge the battery. Maybe I will be lucky and it's just a loose connection.

Scott

I would be willing to bet that you have a grounding issue.  I looked for about three days to find my problem.  But, I fixed it !!