Let's each take a load 825 miles @ $1.30 per mile = $1072.50. We will pay $5 gallon for fuel. You will drive 75 MPH  and get 5 MPG ($1 per mile) and I'll go 55 MPH and get 7 MPG (.71 cents per mile).**

You will get there in 11 hours and spend $825 on fuel for a gross profit of $247.50, divided by 11 hours works out to $22.50 per hour. It will take me longer, 15 hours, but I only spend $585.70 on fuel so my gross profit is $486.8 divided by 15 = $32.45 per hour. Granted I will have a ten hour sleep period, but when I wake up and hit the road it will be at $32.45 an hour and you will still be getting $22.50. Correct me if I'm wrong, but with fuel at $5 gallon it looks like you could earn an extra $10 an hour when you Drive 55.

** "Excessive speed is the largest single factor in reduced fuel mileage. A general rule of thumb is that every mph increase above 50 mph reduces fuel mileage by 0.1 mpg."
Kenworth White Paper on Fuel Economy, March 2006. http://www.kenworth.com/FuelEconomyWhitePaper.pdf