A hypothesis: 75mph is the highway mpg sweet spot

[Aquineas]

I've made reference to my getting above 40mpg with the cruise set at 75mpg, and for me, time and time again it's proven itself to be true. I'm talking about being on a flat road with traffic that's light (or fast) enough that you're not having to slow down all the time due to traffic in front of you. This is with the ICE on, and a perhaps a tiny bit of hybrid assistance. I've noticed that my mileage at 75mph is typically better than my fuel mileage at 65mph. This was true for me before the MPG patch and it remains true now.

[kuzzi]

I see this sweet spot when the engine gets to an optimum temperature. On really hot days and after the interior has cooled down I see the same thing. I think the engine gets to a point of peak thermal efficiency when running at 75 mph on warmer days.

 

I observe this on the bike as well, running at 3800 rpms in top gear will get the ET to ~375F and yield ~40mpg. Running it at the same speed, but at 4500 rpms in 4th gear gets the ET to ~415F and ~43mpg.

[neod192]

My car doesn't behave like that .. I had the cruise set at 70 for 100 miles and I only got 38 MPH. I increased it to 72 for another 180 miles and my MPG dropped to 33-34.

 

On the way back from that trip, I set it to 65 and I got 42.4 MPG ... I would love to get that 42-43 going 75, but anything over 70 kills my MPG ..

 

This is after the PCM update and it just roller over 6k miles so it's all broken in now!

[corncobs]

My car doesn't behave like that .. I had the cruise set at 70 for 100 miles and I only got 38 MPH. I increased it to 72 for another 180 miles and my MPG dropped to 33-34.

 

On the way back from that trip, I set it to 65 and I got 42.4 MPG ... I would love to get that 42-43 going 75, but anything over 70 kills my MPG ..

 

This is after the PCM update and it just roller over 6k miles so it's all broken in now!

Thanks neod192 your post makes me feel better after my for me disappointing trip to Atlanta and Charleston SC.

[djminfll]

I have posted multiple times about how my mileage decreases at speeds over 65mph. This is both before and after the PCM update. And as I posted previously, when we made a 150 mile trip at 65 mph, I got about 45+ mpg, coming home at 70 mph, we didn't hit 40mpg. It is even worse at 75 mph. And this was on an open flat stretch of highway.

[neod192]

I have posted multiple times about how my mileage decreases at speeds over 65mph. This is both before and after the PCM update. And as I posted previously, when we made a 150 mile trip at 65 mph, I got about 45+ mpg, coming home at 70 mph, we didn't hit 40mpg. It is even worse at 75 mph. And this was on an open flat stretch of highway.

I wish I would get 45+ going 65 on flat roads. So far it's not happening. On downhills, I could get 45-47, but on the way back I'll get 41.

[acdii]

35.5 at 80 MPH, 43 MPG at 65 MPH. Pre update of course.

[lolder]

On the interstate, if you're in moderate traffic, the vehicles create a tail wind which is worth 3 mpg per 10 mph tailwind. Drafting within 50 yds of a SUV is worth a couple of mpg.

[Aquineas]

Well gosh folks, I'm at a loss as to the completely different experiences many are getting. I am getting an indicated 40-43mpg pretty consistently on the interstate at that speed, somewhat less MPG when going slower.

[corncobs]

Well gosh folks, I'm at a loss as to the completely different experiences many are getting. I am getting an indicated 40-43mpg pretty consistently on the interstate at that speed, somewhat less MPG when going slower.

Sorry if its in your signature but are you running 17" or 18" ?

[Aquineas]

I'm running 18s (Hybrid Titanium)

[corncobs]

Damn I was kinda hoping you would say 17" that would have given me another explaination but when you see 40-43 are you just by yourself and an empty car or with family and luggage?

[Aquineas]

Saturday I went to/from the Woodlands with the wife and two munchkins (one baby and one toddler). Now on sections where there's a hill, it drops accordingly, and will rise when it's downhill. Perhaps the road is on a modest decline that I'm not visually detecting? But it doesn't explain the return trip. My tactics are as folows:

 

1. Get the car up to speed, not hastily, but not with the cruise control either

2. Set the cruise control when the car is at 75

3. I'm using ACC in eco mode.

 

The whole thing goes south when someone gets me off the pace (which happens pretty often), but when I can hold the speed without interruption, the ICE is on and there's a little blip of hybrid assist going on (see photo in this post).

Edited August 19, 2013 by Aquineas

[corncobs]

Mmhh interesting so let me ask you are also driving the same kinda speed for your daily commute?

 

I still believe that the FFH learns the daily driving routine and provides better FE under those conditions. Don't get me wrong I'm perfectly fine with such a concept since it yields me 50+ on my daily commute but I would hope there would be a parelle option PCM if the car senses a different driving routine like on road trips to adopt to a different driving style faster.

[neod192]

Aquineas, are you seeing 40-43 on the instant only or also at the end of the trip? While I can see 40-43 instant MPGs for some portions of a trip, the trip average tends to be lower.

[kuzzi]

I think lolder nailed it on the head there in my experiences. I tend to drive with a bunch of traffic around me so the entire airflow of the 5 lane road is assisting me. I get the same assistance on the bike. I could be hundreds of yards behind the next vehicle, bunch the general flow of traffic makes the wind buffeting much less on the freeway vs a back country road without any other traffic.

 

I can get 42mpg on a 40 mile trip at 75 mph, but again that's with traffic flowing in the same direction.

Edited August 19, 2013 by kuzzi

[acdii]

I found that flatbed trailers with low loads or a tanker give really good results too. Much less buffeting and a smoother air flow at 1.75 second intervals. Get behind a box van and it knocks you around at that interval and you need to be closer to take advantage of it.

[SteveB_TX]

I wish! My commute is 75MPH most of the way, then drops to 60. Unfortunately, the designer of our highway thought it cute to make it go up, down, up, down, up... you get the idea. Not one single flat spot on I-44 that I travel on. Grrrr. I can keep it in EV for quite a while at 60, but EV only teases me at 75 because the the hills and dales. :)

[Aquineas]

I'll get 41-43 instant, and 39-40 for the whole trip. As far as my commute, I telecommute at least 4 days a week and when I go into the office, I wouldn't take the Fusion as it's the wife's ride.

[hybridbear]

Our experience was definitely the opposite. When we spent a day at 75 MPH driving across Montana and North Dakota we struggled to see 38 MPG. At 65 we saw 42-47 MPG over hundreds of miles. At 70 MPH it was right about 40 MPG. Driving across Montana and North Dakota is actually overall a downhill slope the way we were driving and we still only saw 38 MPG. See my Fuelly fill up number 36.

Edited August 20, 2013 by hybridbear

[Aquineas]

Was it windy hybrid? When I drove east to west across Montana a few years ago it was pretty windy in mid June. Fun fact, when I went in 2004, it was the only time before or since I've set my cruise to 100mph and kept it there for more than an hour. My Acura TL was rock solid at that speed.

[hybridbear]

Was it windy hybrid? When I drove east to west across Montana a few years ago it was pretty windy in mid June. Fun fact, when I went in 2004, it was the only time before or since I've set my cruise to 100mph and kept it there for more than an hour. My Acura TL was rock solid at that speed.

There was wind, but not a lot, less than 5 MPH. I don't remember out of what direction.

[MaineFusion]

The only time I methodically exceeded 65mph was on a long road trip to Kentucky from Maine. In KY where speed limits are 70mph, I set my cruise control to 70mph and I proceeded to lose at least 5mpg. When I returned to 65mph over the same stretches of road on subsequent days, my mpg returned to what it had been doing at the time (about 42mpg).

 

Even after the PCM update, I've observed that if I shift my top driving speed from 63mph to 65mph on my daily commute, I lose about 2mpg.

 

It must be remembered that as speed increases aerodynamic drag increases, thus it requires more energy to offset the increased drag. It should also be remembered that the FFH was optimized to do really well on the EPA fuel economy tests, this meant optimizing it for 62mph, which is one of the primary speeds the tests were conducted at.

Edited August 21, 2013 by MaineFusion

[hybridbear]

 

This shows the impact of speed on fuel economy. At 60 MPH the car will cycle in and out of EV mode, using the generator to load up the ICE to its peak efficiency when running. Above 62 MPH the hp needed to overcome the wind likely made it rare for it to be more efficient to run with the ICE off. At 70 MPH the wind resistance alone is plenty load for the ICE to be operating in its most efficient range. Above 70 MPH and the ICE starts running inefficiently because the load is too high. See this post for more info on what I'm talking about.

[kuzzi]

That chart is dead on. Going 100mph on the bike with no windshield feels like I'm driving through pudding.

 

Pumping out 100+ hp and even though it weighs 600lbs, I would venture it would take all its got to break 130 or 140 mph..