My 1984 GMC K1500, 305, TH700R4, RV cam, rebuilt carb, and Edelbrock intake, does 1300 rpm at 65 in overdrive. I'm assuming that this is due to numerically low differential gearing. I'm getting 11 city and 17 highway. Is this what I should be getting? Due to the large load on the engine, do you think that it would get better fuel economy if I kept it in third? By the way, how do I identify what gears I have?