cabin air filters?

hello, I went to a shop today to get an amplifier set up on my 2003 lincoln navigator. I had about 125 miles in my fuel tank untill it was empty. While installing the ampifier the guy wanted to test the radio and the car wouldn't start for him twice and the car had to be boosted back up. After my amplifier was installed and when I was about to leave the shop it said that I had 345 miles in my fuel tank when I only had 125 before I entered the shop. What could be the problem? Why is the computer incorrectly reading the amount of gas left in my fuel tank? Will this problem fix itself? Is there a way that I can reset the fuel indicator(Miles to Empty)? Thanks in advance and any help with this will be greatly appreciated

I have a 92 Pontiac grand Prix the transmission stoped working no noise nothing i changed the fluid and filter and it worked fine now no reverse. I was told it could be sensor can anyone tell me where that would be like in the transmission or where. Thank's in advance