Highway mileage with my '79 300 with E-58 engine,factory duals,TQ carb, non-lockup 727, 235-75-15 tires and 3:55 SureGrip was 14.7 on a trip to Myrtle Beach last August, trying to stay close to 65 when possible. The air conditioning was running the whole trip. City driving nets about 10 mpg.Engine is running Mopar electronic ignition and a '70 340 camshaft. Current miles on the engine is 224,456.