tatao, My '77 Chevelle which weighs 3700 lbs or so and has the EXACT same engine as your Chevy gets about 20 mpg normal driving and then into the low twenties on a trip. She has the original 3-speed automatic tranny and a 2.56 rear end. They put this high 2.56 rear end in to get better gas mileage as a result of the oil shortages of the early seventies. Your car should get similar performance. There is definitely something wrong somewhere in your car. If it has a heat riser on your exhaust make sure it is working properly. Also install a tachometer and note what rpm's you are turning at say 30 mph and at 55 mph and let us know. If possible jack the rear of the car and with the engine OFF AND JACK STANDS IN PLACE FOR SAFETY count how many drive shaft revolutions there are when you turn the right rear tire BY HAND exactly one full revolution. You can mark the driveshaft with a piece of chalk to help you count. This will give you an idea of your rear end ratio. Also verify your odometer accuracy. One way to do this is compare your readings with a car you know has an accurate odometer, maybe a new vehicle, over the same exact distance. The longer the better. I have the feeling your tranny is not up to par and may be slipping. The tachometer will help you spot a slipping tranny. Also make sure all four wheels are rolling smoothly and not binding. The front wheels should spin freely when off the ground. Also, silly as it sounds, make sure your emergency brake is completely disengaged and not dragging against the rear brake drums as you go down the road. Also make sure all four tires are fully inflated. I keep mine at 30 lbs. You should have miles per gallon figures in the high teens or better. Definitely. And that six should be silky smooth and sing sweetly as she cruises along.
Ken B
