Why My 350 is poor MPG on Gas???

[lost_66]

i want to know why my 82 chevy c/20 engine 350 with SM465 and has bad mileage on gas... i fill up gas for 20 gallion tank cost me almost $30 doller
and i drove on hwy only 65 mph and 2 hour and 15 min later it goes to empty in tank! i figure it was not 17 mpg.. and i dont understand why my engine so bad on gas but there no missing sound and has new tune up wire, spark plug and do you have idea why my engine has bad mpg gas??