Poster,
You should not quote lighter loads less than heavier loads. As Gman said the lighter loads will probably be more valuable cargo. Why would you haul valuable cargo at a discounted freight rate? You shouldnt!! If the shipper wants a full truckload, then they pay a full truckload rate. I once hauled a map in a paper tube from the Navy yard in Bayonne NJ to Oceana VA in a 53 foot van. It paid about $5 per mile and they taped the paper tube to the floor of the trailer and sealed the door. I once hauled 51000 pounds of steel beams from Huger SC to Chicago IL for about $ .90 per mile. My point is heavier loads usually pay less, so lighter, more valuable loads should pay more. Higher Revenue minus Lower Expenses equals More Profit.

My main interest here was obtaining opinions on costs saved per mile hauling light vs heavy. Not too interested in discussion about rates. (although the points made were mostly good.)

The bottom line is that you will mostly notice a little fuel savings. Hauling heavy all the time may put more wear and tear on your equipment, but fuel is where you will notice most of your savings. There are also other factors in addition to weight. Wind is a major factor in fuel consumption. Even with a light load, traveling in heavy winds will trash your fuel mileage. That is one reason why you can't just make a blanket statement about fuel savings when you talk about heavy and light weight loads. For instance, a friend of mine just took a load across country to Nevada. He was in very heavy winds of around 50 mph. He had a light load but his fuel mileage dropped to less than 5 mpg. He usually gets around 7 mpg pulling his flat bed. He pulled a van on this run. I believe he only had about 15,000 pounds on the truck.

GMAN.....I'm not sure why you bother, as it seems you are chasing your tail on this one. He doesn't seem to want input, but rather an answer that can't be provided as it "depends."

You may be right, Flying W. There are a number of new people who come to this board and expect to get finite answers to questions in which there are just too many variables to give an answer that will be correct in all situations. I can understand their frustration, but you simply cannot give concrete answers with so limited information and so many variations to each situation. We might be able to come up with a mathematical formula that would account for weight, wind, speed, driver, etc., but I can get a headache thinking about it.