Gas gauge is caused by a bad ground. This problem is so common I think every car overt thirty years old suffers from it.
Speedometer not working is harder to understand as it is mechanical.
Is the speedometer needle moving eradically acting as though it is catching and then speeding up as you drive at a constant speed. Or is it just not telling the correct speed; but moves smoothly up and down the scale?
If the speedo is just off (reporting the wrong speed) then someone has changed the rear differential gearing to a different ratio than what it had in it when it left the factory. You can determine what gear ratio you have now (by removing the inspection cover and counting the teeth on the two gears inside), and then change the speedometer gears in your transmission (there are two a drive and a driven gear). Another less satisfactory option is to buy a correcting transmission (a housing and gears set on shafts inside a box) that bolts onto the outside of your tranny (these correcting geared transmissions are not only expensive, but bulky, and often get in the way of headers).