My check light is on and my speedometer and mileage gauge.

Gentlemen I am having problems with my 99 Chrysler Concorde LXI. It has a 3.2 liter engine V6 and is 2 wheel drive most likely front wheel. I currently have 131,700 miles on it and haven't done any major repairs on it anyways I will get to the point, recently my check engine light came on and flashed at times. An employee at an Autozone did a engine code read and it says #6 spark plug misfires. I tried to replace the plugs myself except for one thing, A midas store in Minnesota worked on that last year for me before I moved to Arizona. They put new plugs checked the wires and coil to make sure tehy were ok. Their warranty for that work is only for 90 days so I am out of luck for them to look at in an Arizona Midas location. I noticed I can only repalce 3 of the 6 plugs at the time because torqued down the hex screws for the plug wires and stripped the screws. I had to order new ones at a Chrysler Mopar location in my area. Ok today now I noticed my Speedometer doesn't work now stays at 0 but I am clearly going fast in my car like 25-45 miles per hour and now my milleage gauge does not work either what I mean it is staying at the same mileage and not flipping it is an electronic gauge. I don't know what to do guys. Any advice or suggestions you can give me and yeah my gas mileage is horrible too seems like it is sucking way too much gas than normal. Thanks be waiting to hear from you