Highway MPGs Using DRCC: 63.4 at 60+ mph

[a priori]

People have asked: What kind of highway MPGs do you really get? Followed by: Really? At what speed -- 45 mph?

This afternoon I drove from my office outside of Chicago to suburban St. Paul. The first couple of hours I wound through countryside and made a couple of stops in southern Wisconsin. I then got onto I-94 east of Madison and drove Interstate all of the way to St. Paul.

I wanted to see if I could drive at highway speeds, using the dynamic radar cruise control (DRCC) on my Prius V w/ ATP, and get some decent gas mileage. By the way, highway speed to me means the speed limit (or slightly above, if going downhill!).

My theory was that I could get very good results by shadowing (not drafting) 18-wheelers using the DRCC. By imitating the big rigs (gain speed on downhills and bleed it off on uphills), I thought I'd do better than setting the CC at a constant speed and then weave in and out of traffic.

Today, the truckers really cooperated. I shadowed only two different trucks for the last 200 miles (199.7), which is also the period I measured. This is my measurement period because I was driving the entire time on the interstate using the DRCC, and I was able to write down the data starting at 6:24pm and ending at 5:54pm. This gave me roughly 200 miles in 3.5 hours -- not bad, considering I made one poddy stop and had a couple of construction slow downs. (The number turns out to be a bit better than 57 mph -- and that is just going by the clock, even when I was stopped with the car turned off and going through construction slow downs.)

I set my speed at 69mph (speed limit was 65, except in two construction zones where it dipped to 55). Most often I was going slower than 69, because I kept behind (not drafting distance, but far enough to have cars get between us more than once!) a truck that would vary its speed. According to the MID, my average speed during this time was 58mph. Remember this also includes the time to enter and exit the freeway, come to a stop and start up again, as well as go through the two 55 mph construction zones. I'd say I was most often at about 65 while actually driving.

What kind of mileage (fuel economy) did I get?

According to the MID, I got 63.4 MPG. I could say this was at 57 or 58 mph, but the reality of it is that it was at higher speeds. Even though the majority of my time was at 65 mph, I'd accept that my real average for the time I was driving at highway speed was between 60 and 65. To accomplish this across a distance of 390 miles seems pretty impressive to me.

I wonder what difference it would have made had I 15" wheels with 195 tires instead of the 17" wheels with 215 low profile tires (and only 7500 miles on them!)?

Other factors of note: The sky was hazy and a bit overcast. Temps were in the mid-70s, falling to the high 60s by the end of my drive. The land was mainly flat, though the further north I drove into and through central Wisconsin, the hillier the terrain.

"Real" MPGs: Now that I've measured 16+ tanks of gas, I feel pretty comfortable with my estimation that my car's MID reads 5% higher than the numbers I calculate. Applying this to the MID data from this trip (and assuming the odometer miles are faily true), it drops the very impressive 63.4 MPG to 60.4. That's enough for me to say that I can still get great gas mileage while driving on the highway:

Even though the car reported my aveage speed at 58 mph, I am willing to say that my actual driving time was less, therefore my speed greater. I estimate my real driving speed to be in excess of 60 mph and as high as 65 mph.

What does it mean? I am now very comfortable telling people the DRCC is more than just a cool toy. By mimicing a truck driver's actions, I was able to get better that 60MPGs. It really means that fuel effiency may be right around the Corner!

[ALS]

I wonder what difference it would have made had I 15" wheels with 195 tires instead of the 17" wheels with 215 low profile tires

It would only have made about 1-2 mpg difference.

[tumbleweed]

What were your tire pressures? Did you use the AC? If you did use AC were you in the ECO Mode? Did you use 3 bars the whole distance? I find on the freeway I sometimes need to drop down to 2 or even 1 depending on traffic conditions.

I checked my mileage from here to Portland on I 84 using DRCC, about 200 miles each way. I had my DRCC set at 65 to 70 except the 20 miles or so when I'm near Portland then I drop it to 60. Last trip was about a week ago, AC on most of the trip, ECO mode, tires at 48 lbs. There are quite a few hills, no mountains. To many hills to be very sucessful following trucks as you were. The wind was blowing 30 Knots or so and I only got 46 MPG (displayed) going West. Coming back I over the same route I had a tail wind and managed 53 MPG displayed. Previous tests indicate my displayed mileage is about 2 to 2 1/2 MPG better than actual MPG. I think my Gen2 would have gotten about the same mileage.

So maybe your trick with the trucks works pretty well, your mileage is certainly better than mine. But I don't think it will work around here because the elevation changes slow the trucks enough to hurt mileage.

I'll be going over the same route again next month and I'll try to find some cooperative trucks and see what happens, maybe the wind won't be blowing and I can get some better numbers.

[royrose]

Originally Posted by a priori

This afternoon I drove from my office outside of Chicago to suburban St. Paul. The first couple of hours I wound through countryside and made a couple of stops in southern Wisconsin. I then got onto I-94 east of Madison and drove Interstate all of the way to St. Paul.

Are your figures for round trip or one way? Definitely useful and interesting info but one way never tells the whole story.

Tumbleweed: I've driven that route. Is the wind ever not blowing?

[hobbit]

For starters, doing the bleed-off-on-uphills and get it back on
the downside *is* an efficient way to drive, and every trucker
knows this [whether they actually practice it or not, subject to
their engine power]. But more importantly, even if you had DRCC
on the "long" setting which the manual says is about 160 feet, you
were less than 2 seconds behind the truck at 88 feet per second,
and therefore drafting trucks at a distance that the truck drivers
*really* don't want you to be at. My outside voice at this point
wants to say "knock it off", but for the most part you're only
endangering yourself in that situation. And setting a bad example
to those around you.
.
Try this next time: mimic what the truck *would* be doing over
the same terrain, by locking your foot at a particular HSI demand
and letting things play themselves out but keeping an actual
*safe* following distance from other traffic. Your results
will likely be similar, but I'd be interested in which way
the long-term delta is -- on one hand you maybe get a bit of
aerodynamic help by the tailgating trick, but on the other you
don't have to react instantly to any change made by the vehicle
ahead and can thus ride right in your own sweet spot more
effectively by leaving a nice big open "cell" to play in.
.
Sorry, DRCC and LKA are *not* substitutes for 100% aware driving.
Not even close. Maybe when a tire cap comes in through your
windshield, you'll understand.
.
_H*

[tumbleweed]

Originally Posted by hobbit

For starters, doing the bleed-off-on-uphills and get it back on
the downside *is* an efficient way to drive, and every trucker
knows this [whether they actually practice it or not, subject to
their engine power]. But more importantly, even if you had DRCC
on the "long" setting which the manual says is about 160 feet, you
were less than 2 seconds behind the truck at 88 feet per second,
and therefore drafting trucks at a distance that the truck drivers
*really* don't want you to be at. My outside voice at this point
wants to say "knock it off", but for the most part you're only
endangering yourself in that situation. And setting a bad example
to those around you.
.
Try this next time: mimic what the truck *would* be doing over
the same terrain, by locking your foot at a particular HSI demand
and letting things play themselves out but keeping an actual
*safe* following distance from other traffic. Your results
will likely be similar, but I'd be interested in which way
the long-term delta is -- on one hand you maybe get a bit of
aerodynamic help by the tailgating trick, but on the other you
don't have to react instantly to any change made by the vehicle
ahead and can thus ride right in your own sweet spot more
effectively by leaving a nice big open "cell" to play in.
.
Sorry, DRCC and LKA are *not* substitutes for 100% aware driving.
Not even close. Maybe when a tire cap comes in through your
windshield, you'll understand.
.
_H*

Hobbit, I share your concern about adequate following distances but the owner's manual is a bit misleading on that point. The 3 bars = 160 foot distance is only at 50 MPH. The bars are more closely related to time, 1 bar = 1 second, 2 bars = 2 seconds, and 3 bars = 3 seconds. It's not exact but it seems very close, I've checked it a number of times at speeds from about 40 up to 80 MPH. 3 bars equal to 3 seconds also means that the 160 feet at 50 MPH is wrong, it should be 220 feet. Perhaps others who use this system could confirm my observations, please? I would speculate that the software that controls the system may have been changed after the manual was written.

The DRCC also seems to pay very close attention to the speed differential. If the car in front slows rapidly or if you approach a slower vehicle rapidly it will brake rather sharply to compensate and maintain a proper distance. On the other hand if the car in front is accelerating away from you, such as when someone passes and cuts in front, the DRCC may not slow you down at all, depending on how close it is, and just allow the distance to open up.

But like any other vehicle control system it needs to be monitored because it could make a mistake. After 6500 miles, mostly on the freeway, mine has not gotten anything wrong yet but I watch it closely and I always will.

[nparker13]

Good to know about the DRCC. Unfortunately the highway I take to work doesnt allow trucks, and is very hilly, so I tend not to use it, but on long trips like the one I'm about to take tonight, I will definitely be more inclined (pun intended) to use it even uphill.

[tumbleweed]

Originally Posted by royrose

. . . . . . .
Tumbleweed: I've driven that route. Is the wind ever not blowing?

Most of the time it is Roy, especially in the spring. Good place for wind turbines though.

[Shawn Clark]

Originally Posted by hobbit

But more importantly, even if you had DRCC
on the "long" setting which the manual says is about 160 feet, you
were less than 2 seconds behind the truck at 88 feet per second,
and therefore drafting trucks at a distance that the truck drivers
*really* don't want you to be at.

Let's be realistic. That distance is an eternity behind a truck or even a car on many roads. You will have cars weaving in between you and the truck at that distance. There is little for the trucker to worry about with you at that distance. The truck won't out brake you. And they don't seem to have trouble changing lanes right in front of you when you are passing... When you get locked in traffic there will be several cars between you and the truck at that distance.

One should be more concerned about following sporty vehicles with short stopping distances. Whenever I have to brake hard I'm usually watching my rear view mirror to determine if I'm going to have to instead take evasive maneuvers to avoid being run over. There's nothing quite like the thrill of seeing an 18 wheeler barreling down a hill at you smoking his tires because some jackass stopped at the bottom of a hill in the highway. And while you ponder taking the ditch, having the car behind you beat you there...boxing you in so that he can avoid being run over. Leaves an impression even when the trucker does manage to shut it down a foot off your bumper.

Where folks get into real trouble with truckers is side-by-side driving, especially in their blind spots. It gives them no room to maneuver and they might not be aware of you. The other problem is folks that pull right in front of them. The trucks need more room to stop.

[ken1784]

Originally Posted by hobbit

But more importantly, even if you had DRCC
on the "long" setting which the manual says is about 160 feet, you
were less than 2 seconds behind the truck at 88 feet per second,

Just FYI,
The 160 feet distance is at slow speed, say 30 mph.
The set distance is automatically increased as speeding up, say 250 feet or more at 60 mph.

Ken@Japan