Originally Posted by bob5560g
Thanks for the info. What was your usual cruising speed on the interstate?
I'm getting 1 mpg less at around 13.5 at 65 mph. Here's some extreme data. Speed limit across west Texas is 80 which gave me 9.8 mpg. So as a test I slowed down to 55 for the next tank and got 15.0. I think the wind resistance at the higher speeds is a mileage killer.....
My step-brother is an engineer working R&D for GMC. We got to talking about the 3 liter V6 Turbocharged diesel engine along with the 5 speed auto that Mercedes-Benz uses on their Sprinter 3500 chassis.
Diesel engines are designed to provide the best pulling power along with fuel economy when within the maximum Torque range. That combination is between 1400 and 2400 rpm. At 2400 RPM which is at the top of the torque range our Sunseeker cruises at a cool 60 MPH.
As the rpm's go up that torque goes down BUT the Horsepower increases. It takes calories (fuel) to produce Horsepower. At 70 mph we are turning 2800 RPM which would obviously burn more fuel, running 80 mph would use even more.
Obviously you would be loading down the engine if you try to run in fifth gear at 1400 rpm so you would want to be in the 2000 to 2400 rpm range when running in fifth gear which would equate to optimum fuel economy running down a level, at sea level road, with no head wind in the 50-60 mph range.
Basically if you are into pinching pennies and don't mind being run over by semis, then running 50 down the expressway would be the thing to do. Here in Michigan the truck speed on the interstate is 60 mph and most run 65. I choose to run with them and not worry about the gallon or two a mile I may lose instead of being run over by them.