[edpontiac91]

Originally Posted by Pampered-Z

Also the maintainence and other factors will have a big effect. My DD is a 99 GMC Z71 PU with the 5.3. I get my best highway mileage ( Driving the PA TP over 100 miles each way weekly, if I stay between 55-70MPH ( 70 MPH is 2000 RPMS ), if I exceed 70 my mileage drops noticabley. I assume either convertor unlocks or just the air resistance?

Originally I was getting around 19 highway, I installed a K+N filter and tanoe cover and milage went up to 21-22. When I changes tires to a HD type for towing, the tires weight about 7 pounds move then the previous ones, listed as the same diam., and I use a slightly higher air pressure, and my mileage dropped to 20-21. Since the truck now has 165K on it I've swtich from 5-30 to 10-30 high milage oil and it dropped to 19-20MPG. Next oil change I'm going to go back to 5-30 just to see what effect that has.

You're right about the WIND RESISTANCE. Above 65 mph, you start to run into the fact that your truck is a brick and faster you try to go the harder you have to push on the gas. Going from 30 mph to 50 mph is easy, but trying to go from 50 mph to 70 mph will require a lot more effort on the engines part. Also you may well benefit from a SYN motor oil(such as Mobil 1)with a 5W30 rating. It holds up better under heat and higher loads and should give you a gas mileage boost. You can also go further between oil changes(like 5,000+)so the cost factor evens out when compared to regular motor oil. Those HD tires have a tread pattern that is more aggressive and does not roll as easily on the highway, again it affects gas mileage.