driving faster saves more gas?

I have noticed something similar. Yesterday, I tried a different route to work. It kept me off the interstate and on mostly two lane roads, passing through several towns and traffic lights. I got 54mpg. Today, I went back to my normal, interstate route where I cruise at 65 mph, and I averaged 59 mpg. It seems to me that keeping the ICE operating at a fairly constant speed near its sweet spot gets be the best numbers.

As an aside, these last two days we've finally had a warmup out here on the east coast. I had been struggling all winter to keep my mileage above 50mpg. Warmer weather got me a nearly 20% increase in mileage!

 

I'd be hard pressed to believe that driving a sustained 70+mph would improve MPGs. Maybe your test results are influenced by your ability to P&G, use hills or your experience with a Prius. When I'm on the interstate at a constant 65 MPH, my mileage takes a hit by as much as 5 mpg. I get my best mileage at lower speeds.

 

It sounds like you're using Super Highway Mode. People with a Scangauge seem to be able to identify it by looking at RPMs and ignition timings, but I don't have one, so I identify it as "45-55 MPH, HSI bar a few blips above the halfway mark" It's one of my favorite ways to travel because it's efficient without varying my speed too much or requiring a whole lot of monitoring.

 

I have noticed that the best mpgs seem to be
about 48 - 50 mph. I noticed an indicated mpg of
about 70 mpg consistently. Which is about 65 mpg true mpg.

Now, this is during warm weather, 70-80 degrees, and dry roads, and no wind. However up here in the Northwest you only get
about 2 weeks a year with that type of weather.

al

 

On my daily commute route, I have a short 55mph hwy section. With a full battery and the afternoon tailwind, I can manage a constant 75mpg on this section.

I get up to 60mph, lift off the throttle, and slowly feather it back in till the HSI bar is just to the right of the ECO lettering. By that time, I'm in the 54-56mph range. 51-53mph is even easier to maintain the 75mpg avg. The SOC doesn't seem to change while doing this time (3-4 miles). I assume this is what is referred to as Super Highway mode (?).

On the same road heading the other way into the headwind, I'm barely above 50mpg.

 

Super Highway Mode can probably be used when the speed limit is 40-65 MPH (perhaps speeding a little on the low end, perhaps a little under the limit on the high end).

Freeway speed limits are generally higher than that (70 MPH here), so it's harder to apply in that case. However, some people like to use it on the freeway by "pulsing" gradually to freeway speeds and then entering Super Highway Mode, losing speed in SHM, and repeating. That's what sipnfuel is talking about in this thread. As far as I can tell, it works fine, as long as you don't "pulse" hard.

 

Ohhh...why didn't you say so...I can agree with that; sort of.

 

Accelerate hard and slow down slow. That's all i do.

 

I find this unlikely. My guess is that if the end of the world came and your life depended on maximizing mileage you'd probably want to be at 30-40 mph. Drag at speed increases exponentially; going from 30 to 40 requires less additional effort than going from 40 to 50. Highway driving is only better on most other vehicles because city driving is so terribly inefficient (no regen) but highway is still innately power-hungry.

 

a big +1 here.....staying off the freeways and keeping speeds in the 35-49mph get me about 65mpg in warm temps (70+ degrees).

 

I wonder if these results are due to the specific route and road conditions you are working with. A very slight slope could make a big difference in your results. The 36mile freeway trip from my house to Davis does not appear to be downhill but it is and my fuel gauge and MFD tells the story. On the trip there I can get 58mpg at 65mph but on the way back I drop to around 50mpg.

Anyway, not saying you are wrong but it goes against physical laws unless the car is doing something to increase efficiency beyond the losses from moving air.

 

I'm no expert, but this seems be highly related to the reason pulse & glide (P&G) is more efficient than steady driving. Pardon if this is all obvious and I come across as talking down to anyone, but here's what I think is happening:

In an ideal frictionless environment, maintaining a constant speed would require no power at all. In the real world, we of course have resistances that need to be overcome, but at low speeds, some of these resistances (especially air) are minimal, which means that it takes less power to simply maintain a constant speed. At higher speeds, resistance is stronger, so maintaining a constant speed requires more power. And of course, this is why it is easy at first to think that higher speeds should result in lower efficiency.

But given that combustion engines have ranges of RPMs where they perform optimally, and anything below (or above) this optimal range will have lower power output per unit of fuel, on any car there will be a minimal speed below which any attempt to maintain constant speed will operate the engine outside of its efficient range. Based on Hobbit's HSI tutorial (3rd image), it appears that this speed corresponds to the HSI bar just under, maybe a bit to the right of the 'O' in ECO, as you have noted yourself. Whatever speed can be maintained on any given day and route while simultaneously maintaining the HSI here will be close to optimal, assuming power is applied to the wheels in a perfectly steady manner.

I've found this speed to be around 50-something mph myself, which is again in line with what you've noticed. Maintaining 30 mph would require holding the HSI over in the left half, which would either involve running on pure battery until the car decided to use the engine to recharge the battery, or running the engine below its optimal RPM range. Both of which are inefficient ways to maintaining a steady speed.

Now if you were going 30 mph and tried to simultaneously operate the engine within its optimal range, you'd of course find yourself accelerating rather than maintaining a steady 30 mph. Once you decide that you've gone too far over 30 mph, you will probably back off (don't want to get a ticket!), allowing the engine to shut down and not using any (or as much) battery power, since you're trying to slow down at this point. Eventually, you'll be going slow enough again and decide that you want to operate the engine within its optimal efficiency range again, so you give it some gas and start accelerating. And voila, we have the discovery of P&G. So while in the previous paragraph, we were considering ways to maintain a constant level of power to the wheels, in this paragraph, we are considering ways to maintain an average level of power over time, but at any given moment, power to the wheels might be higher (in the optimal engine range) or lower (preferably 0) than that average.

 

I still believe that a prius at 30 mph will get better mileage than one at 50. If anybody had a track or a business park they could set cruise at these speeds and figure it out for sure. I don't have such conditions so cannot say either way. But air resistance at 50 is huge compared to 30.

 

I believe Bob has charted this out already and he is pretty meticulous with tests.

 

Has he? I see this thread, but that seems to be for 50+ speeds. Then there's this thread, where both Bob and Wayne post some interesting graphs. I have a hard time deciphering all of those fancy plots, but it looks like there's an MPG peak at 20 MPH. I don't understand how this is determined, given that the ICE doesn't have to run at 20 MPH.