Went on a long drive today with the family towards east Texas. On the way back decided to run a personal (unscientific) MPG experiment. I set the cruise control at 79 mph, 74 mph, 69 mph and then 60 mph and went for 50 miles at each of these speeds. My results: 50 miles cruise control @ 79 mph ===> 43.5 mpg 50 miles cruise control @ 74 mph ===> 46.1 mpg 50 miles cruise control @ 69 mph ===> 49.5 mpg 50 miles cruise control @ 60 mph ===> 58.1 mpg Here are the parameters: 2008 Prius 10500 miles OEM tires inflated to 42/40 psi Today's high temperature 91'F Regular 87' octane gasoline Only has to disengage cruise control for about 30 seconds - the effects of which I choose to ignore. I wish I could do the 64 mph experiment as well but I only went about 220 miles away and it did not occur to me on my way there to do this But this was certainly eye opening. Its basically a : 33.5% fuel economy improvement slowing down from 79 mph down to 60 mph 13.7% fuel economy improvement slowing down from 79 mph down to 69 mph 5.9% fuel economy improvement slowing down from 79 mph down to 64 mph No big discoveries/news here - just a personal verification of a well known fact. Seeing is believing ... And I realize that this is quite unscientific - but its good enough for me and perhaps some other people might find this pseudo useful. Cheers!
Thanks, jamgolf. It's always satisfying when experience confirms what you think. Speed is the enemy of fuel efficiency.
Thank you very much! I've added your data to this graph: For reason's I've never understood, I've not found anyone else who plots actual, measured, MPH vs MPG yet the data is so terribly useful. <sigh> Bob Wilson
Power to overcome aerodynamic drag increases by the speed cubed. In other words, it takes eight times as much power to go twice as fast, just looking at aerodynamics and the faster speed (power = speed * drag, drag ~ speed^2). Tom
Thats a nice experiment with some great numbers you got! Out here in the bay area either the traffic barely moves (bumper to bumper) else the it moves way too fast (in excess of 75 mph). I wish i could add some more terrain into these numbers. Cheers!
OK, I did my own experiment this weekend, although not nearly the detail. 2008 Prius, has ~2000 mi. Drove from Houston to Waterwood (Lake Livingston) and back - 96 mi each way. Myself and two passengers (wife and 10 year old). From Houston: 93 F outside, A/C set to 77 inside. On the way up, drove between 60-65 mph. Accelerated down hills and approaching up hills in order to build momentum. Coasted up hills to avoid uphill acceleration. Result: 53.3 mpg To Houston: On the way back, had sister-in-law along (so had myself and 3 passengers). So to avoid inquisitive remarks by her about my driving, I figured I'd drive the way I "used to". 91 F outside, A/C set to 76. Drove 65-70 mph, avoided using any hypermiling techniques. Result: 45 mpg. Calculated from the MFD. My experience so far is that my actual mileage is about 2 mpg below the MFD. So yes, slowing down did have an effect! Although I was hoping for slightly better numbers than this. I get well over 50 going to/from work (half highway, half 35/40 mph roads with stoplights about every 1/2 mile or so. Try to pulse/glide and coast to stoplights whenever traffic allows, which is not often in Houston!).
At 65 mph, I expect ~52 MPG and at 70 mph I would expect ~49 MPG on cruise control on I-20 at 70F. Your numbers are well within the expected range under the higher temperatures and AC load. There have been some investigations of hill climbing speeds and so far 55-60 mph seems to be best at avoiding inefficient engine power settings and drawing on the battery. Above 65-70 mph the engine has to work at inefficient power ranges when climbing a hill and the vehicle will draw energy from the battery. One of the best ways to drive hills is to follow a heavily loaded truck at a safe distance. Just match their speed profile. Bob Wilson
