Well I'm tickled pink over the new 2011 3/4 ton denali gmc "Usually" I try to just set the cruise around 65 and go. It has things unheard of automatic just a few years ago. from stibilitrac that powers opposing wheels as needed to stabilize sliding on wet or icy surfaces to computer control anti sway "will apply any one or more wheel brakes independently to correct for excessive trailer sway, and an exhaust brake linked to the computer and to the variable veined turbo charger. a 6 speed allison tranny that will auto drop a gear to reduce for down hill. A bunch of goboldy gook for "set cruise and it WILL stay on speed...up hill / down hill / round corner etc etc etc
BUT... I found its not always the best to set cruise in hilly or mountainous drives?!?! I like to get a little "RUN" at the next one, by allowing it to gain a bit extra on way down from last, and I will allow it to slow down a bit rounding top (I try not to get to excessive with this, or annoy other drivers) I am sure this adds to fuel mileage, less heat build up in engine and brakes, less wear and tear on TV
2012 Cedar Creek 36 RE Touring ED, all avail options
2011 GMC 2500 HD Denali DMax 4wd Superglide firestone bags
95' Procraft V-180-C 120HP OB, 80# cust TM, side/down image sonar
Don and Rosie, Annie the wonder Dog clicky to our photobook album
Running 55 - 60 is nice if you are only traveling a short distance. However on some of our trips (800 plus miles in a day) it would cost use more. We usually run 73 -75 mph and average 62-64 which is a 13 hour day. At this speed, my ford 6.0 gets 10ish pulling a 34 ft triple slide 5th wheel. If the average speed is proportion running 60 mph is a 50mph average or a 16 hour day. Based on everyone response, I should get 10% to 20% more in MPG. The difference for me is $50.
Last summer we drove from Indiana to Nova Scotia (1600 miles) and 2010 drove from Indiana to Glacier NP (2000 miles). Adding three hours to each day's drive would add two days to travel time on these trips. This added time would cost more then the extra fuel; there is food lodging etc.
To be sure, you have to actually measure your gas mileage under different conditions/situations to see what actually happens. I'm one of those types that likes to keep track of my mileage on spreadsheets, and have a scangauge and other real time indicators, none of which work all that well. The scangauge really has consistancy issues with my cummins diesel.
With my 2005 4 cylinder malibu, the difference between 55 and 80 is maybe 1 mpg at best. They're so close you're not sure if you're just measurement errors, or actual differences. The trip computer in that vehicle is also pretty good, generally within 1 mpg of hand calculated values. With my 2011 6.7 liter Ram, and gasser truck I had before it, the difference is significant, towing, or unloaded. My truck has negligible difference between 55 and 65 unloaded, but drops off fast after 65. Towing, the sweet spot seems to be between 55 and 60 and things go downhill in a hurry above that. The difference shows up quite clearly in both the trucks mpg meter, which matches up fairly nicely with hand calculated measurements for the whole tank of fuel.
All bets are not off if you are headed into a strong headwind. Wind resistance is indeed exponential with wind speed. If you are driving 55 into a 20 mph headwind, you will have the wind resistance of driving 75 mph and can be calculated from that speed.
I made the mistake last fall of spending a day dragging my TT across Oklahoma into a 50 mph headwind. I was barely able to hold 55 mph. On the one hand it was nice to know my TV COULD drag the trailer that fast (ie, 105 mph wind speed) but it completely wrecked my mileage that day. It was horrifying to watch the fuel guage drop as though I had sprung a leak.