I get about 12 to 13 mpgs driving around town but then i hop on the highway and i see my gas gauge drop about a quarter tank in about 3 miles and it doesn't rebound going 55 to 65 depending on traffic anyone have an idea of whats going on

 

Please give year and engine size, EFI or carb etc.

Could be debris in the tank or a bad spot on sender. Do you have dual tanks? If so sometimes a bad selector valve will allow fuel to be pumped from one tank to the other.

 

Year - 1984
Engine - 302 V8
Carb - 2B with spacer plate

 

Single Tank so would i have a sender and if so where

 

The sender is in the fuel tank.
Here is an example but don't know if this is the correct unit for your truck?

http://broncograveyard.com/bronco/i-...ord-truck.html

If the sender has a 4 wire connector then there is an electric fuel pump attached to the sender. If it has 2 wire connector like the one shown then there is no pump in the tank.

 

and what would cause it if you only have 2 wire connector

 

Could have the same problem with either type. I was just giving you info so you would know which type of sender to get if you need to buy one.

How often do you get the erratic readings?
Some possible causes:
-The float on the sender could have a hole in it and is saturated with fuel.
-Could be a gummed up or worn spot on the resistive element giving false readings.
-The pivot arm on sender could be hanging up on something.
-Debris in the tank.
-The dash gauge or associated circuits could be bad.
-Wiring from the tank sender to the gauge.

 

Basically Everytime I get on the Highway I Get About 7-8 MPGs but i get 12-14 MPGs in City

 

sounds like you may need to rebuild the carb, or adjust it

 

probably adjusting since i just had it rebuilt like 3-4 months ago