1990 Mercury Cougar my speedometer and gas gauge do not wor

I bought this car from my step mother a couple of months ago, she had quit driving and the car was cranked a couple times a week, it was not drove a whole lot.
The gas gauge never has worked, so I just used my trip-meter to know when I needed to fill up again, a couple of weeks ago the speedometer went haywire. It would go up to 50 or 60 mph and then down to 30 or 20 just random oddball speeds, it finally just all together quit, since this does not work, I can-not keep up with my mileage. I live alone and have to drive 80 miles one way to take Chemo for Cancer, i'm kinda scared to get to far away from home. Is this in the computer part -oh by the way it is digital and total electric. Everything else works just fine, what can I have checked to see if these problems can be fixed. Maybe it could be as simple as a fuse or circuit. It would not let me send u a pic.