1993 ford ranger gets only 17 mpg on highway what can be the cause

If you have bigger tires or wider tires, it will increase surface area and your engine will work harder to get the car moving and will decrease mpg. If your engine could be running rich. If it is injected, look into getting your injectors cleaned. Change spark plugs of they are worn down. Check your oil level and condition. You could also remove the serpentine belt and check that your pulleys all move smoothly. Make sure your AC clutch is disengaging when you have defrost and AC off.

I hope it's not the head gasket going bad , poor mileage is an indication .

Do a tune up and see if it improves