[Viking]

Hi,
I have a Prius on order but have discovered that a couple of the rental companies have managed to secure a few Priuses. I am trying to secure one for a 2500 mile trip next month that should be a terrific test drive. The majority (90%)of the trip will be Interstate highway driving so my question is ...will it be better to use cruise control, even in hilly areas? I am guessing that 55 mph would be the sweet spot for the fuel efficiency though 60-65 will probably be more realistic to go with the flow of traffic. Perusing through the site I have found the tire pressures, ty, and would also like the type/brand of oil it requires even though I suspect it won't be required. Any other hints/ideas to get the most out of the test drive would be appreciated. Thanks again

 

[Brian]

I just got back from a trip to Prescott, Az and used cruise control 98% of the time. 90% of the time I was going 80mph and at that speed, I still got 53mpg.

 

[Viking]

That's very impressive, and at 80 mph, no less!! Was that with a tailwind or using any drafting techniques? Obviously, cruise control is the "way to go"....ty

 

[patrickg]

That's very impressive, and at 80 mph, no less!! Was that with a tailwind or using any drafting techniques? Obviously, cruise control is the "way to go"....ty

Yeah! I'm impressed by the mileage at those speeds. (TAILWIND? or maybe DOWNHILL?)

I haven't finished my tests yet but preliminary information suggests that my personal surmise regarding cruise MPG vs manual MPG is likely right. My surmise is that due to the aggressive nature of the scaling/feedback in the cruise on the Prius results in poorer economy than that of an alert driver using manual throttle control. This is in "rollercoaster hills" with a set speed of 65mph. When I start up a hill speed drops off a couple mph and the cruise jams on the throttle to compensate only to overshoot by a few mph as the hill levels off prior to starting down.

Exercising manual control, I typically have nearly as large a fluctuation from "set" speed as with cruise but not in the same way. Since I can see the hill I can anticipate and the computer can't. I can let the car lose a few mph (and keep consumption low) as I approach the top of the hill in anticipaton of regaining the speed on the downhill side. I estimate that I an averaging the same speed as the cruise would but with less hard acceleration up hills with the resulting overshoot in speed. Excursions above set speed have mileage penalties.

I do not submit this as a practical means of increasing mileage, just the preliminary results of my tests. Setting cruise and not worrying about it, even in "rollercoaster" hills does not have a big penalty. Also if you are concerned that I am impeding traffic in any way while "playing", I am driving on secondary state highways where traffic is very light. It is not uncommon to go for several minutes without seing another vehicle going either way.

Pat

 

[BIF]

I think I can anecdotally confirm Patrick's assertion that an alert driver can have a slight improvement over the Prius cruise control.

I only use CC when my foot or my attention span need a break. And I always notice that the instant MPG figure drops like a stone when climbing hills under CC.

I don't have such a big hit when I am in control of the throttle during an ascent. In fact, if traffic permits, I'll keep my foot steady and let the car decrease in speed. My consumption stays pretty much the same, and I regain my lost speed after I crest the hill.

 

[ehurtley]

I just got back from a trip to Prescott, Az and used cruise control 98% of the time. 90% of the time I was going 80mph and at that speed, I still got 53mpg.

Prescott, you say? Various roads to I-40 to 89, or I-10 to I-17 to Hwy 69? The former is less hilly, and has lots of steady truck traffic. (And few cops.) The latter is hilly, more erratic traffic, and more police. I'm guessing the former.

(I went to Embry-Riddle in Prescott, and drove from Portland, OR to Prescott and back many times.)

 

[Daniel]

I am going to suggest that the highest mileage for the Prius is much slower, maybe around 45 mph. Conventional cars get their best mileage at higher speeds because their engines are less efficient at slower speeds, but Prius has solved that problem with HSD and remains efficient at slow speeds. On the other hand, wind resistance increases with the square of speed, so the faster you go the lower your mileage will be.

I've only taken two road trips. At 75 mph I get around 44 mpg, but at 55 mph I get 50 mpg. I always use cruise control. I'd rather have a relaxing, comfortable trip than squeeze one more mile out of each gallon of gas.

I recommend driving at the speed limit, so as not to be a nuisance to the drivers behind you, rather than driving slower for the increased mileage. But I'd also recommend a strictly-enforced 55-mph speed limit as part of a rational energy policy.

 

[patrickg]

I am going to suggest that the highest mileage for the Prius is much slower, maybe around 45 mph.

I recommend driving at the speed limit, so as not to be a nuisance to the drivers behind you, rather than driving slower for the increased mileage. But I'd also recommend a strictly-enforced 55-mph speed limit as part of a rational energy policy.

When I was last in SOCAL(late spring) the average freeway speed when traffic would permit was at least 85 in posted 65 areas. Driving the speed limit could be hazardous to your health. I can average 50 mpg at 60-65 mph, most of the time on the interstate. The posted speed where I did this was 70. Since the Horse Power developed is the main variable in fuel consumption and the HP required to maintain a speed is proportional to the cubed speed not squared, the difference between 60 and 70 mph makes a noticable difference. (I am NOT refuting the aerodynamic drag being proportional to square law.)

I won't sacrifice safety in the form of driving way slower than most of the traffic to make a mile or two more mpg.

Figures posted elsewhere on this forum suggest that Prius mileage is better well below 45 and that is reasonable (as an assumption not a recommended strategy on the highway.)

I lived through the double nickel (55 mph) national speed limit and its repeal. Try to find a political candidate who will come out in fovor of restoring it. Try to get sufficient signatures to get it on a ballot as a referundum. Politically it is equivalent of the "3rd rail, touch it and die.

There is what makes sense and there is what you can legislate and the two don't cross paths a good deal of the time. When even on a forum dedicated to a "GREEN" economy car you have considerable support for drag racing, RADAR detectors to protect unlawfuly fast driving, folks bragging about going 107, etc, think what the mind set must be for just regular folks. We are becoming scofflaws.

Pat

 

[karenann]

Patrickg wrote: "I haven't finished my tests yet but preliminary information suggests that my personal surmise regarding cruise MPG vs manual MPG is likely right. My surmise is that due to the aggressive nature of the scaling/feedback in the cruise on the Prius results in poorer economy than that of an alert driver using manual throttle control. This is in "rollercoaster hills" with a set speed of 65mph. When I start up a hill speed drops off a couple mph and the cruise jams on the throttle to compensate only to overshoot by a few mph as the hill levels off prior to starting down.

Exercising manual control, I typically have nearly as large a fluctuation from "set" speed as with cruise but not in the same way. Since I can see the hill I can anticipate and the computer can't. I can let the car lose a few mph (and keep consumption low) as I approach the top of the hill in anticipaton of regaining the speed on the downhill side. I estimate that I an averaging the same speed as the cruise would but with less hard acceleration up hills with the resulting overshoot in speed. Excursions above set speed have mileage penalties."

I READ THE MANUAL AND I THOUGHT THAT IT INDICATED THAT USING CC CAN REDUCE MPG. I FIND THAT CC ON ANY CAR GIVES ARTIFICIAL EXTRA GAS BURNING ENERGY BOOSTS. I GO MANUAL AND GET ABOUT 50 MPG WITH EACH FILL-UP.

 

[Viking]

It appears that the cc on the Prius has the same operating limitations as normal cars do....that is....too aggressive at maintaining the speed when climbing hills but does fairly well on level highways....So the the best compromise is to do the hills manually and re-engage the cc when the cruise speed is reattained on the downslope or when at a constant speed again on level ground. I thought that the Prius would perhaps have had a higher/faster sampling rate to compliment the car...How are you all getting "equal" or consistent fill ups of the fuel tank so the mileage calculations are accurate? Are there any tricks or techniques involved with that....TIA plus ...I sure appreciate the information previously posted!!

 

[karenann]

Viking writes: "How are you all getting "equal" or consistent fill ups of the fuel tank so the mileage calculations are accurate? Are there any tricks or techniques involved with that...."

You learn to watch the right side of the "consumption" screen. You quickly can learn how to "lighten up" on the gas pedal so that you can "coast" (stealth driving) and raise your MPG ratio. It becomes a bit anal-- but it pays off in fairly consistent MPG. I try REAL hard to maintain my 50 MPG per fill up.

Incidentally, I fill up every half tank because I did that for years with my "regular" car. I try to do most of the things that John talks about on his own site. We should be doing these things also with our regular cars.

 

[CaPriusLover]

I drive a nice roller coaster route each day on my way to work and use CC all the time. My MPG at fillup is between 46 and 47. I'll start using manual mode for the next couple of tanks and see what I get. If I can get 50, I'll stop using the CC ... at least most of the time!

 

[Daniel]

When I was last in SOCAL(late spring) the average freeway speed when traffic would permit was at least 85 in posted 65 areas. Driving the speed limit could be hazardous to your health. <...> I won't sacrifice safety in the form of driving way slower than most of the traffic to make a mile or two more mpg.

I agree that driving way slower than the flow of traffic is a bad idea. But I also think that 85 is excessively dangerous, and I'll risk offending folks by saying that any place where the average speed is 85 is populated by lunatics.

The speed limit on the freeways outside of town around here is 75, and I'd say that's about the dividing line between risky and insane.

I, too, lived through the 55 mph limit. We still got where we were going, and fewer of us died in the attempt.

 

[ehurtley]

Hrm. Now that I'm thinking about it, you would think Toyota could have designed a more intelligent CC for the Prius. One that may be a little less aggressive on the acceleration, that doesn't automatically kick off if it can't hold the speed, etc. Another feature I'd like is for it to use regen /engine braking to SLOW the car down automatically when going downhill to maintain your CC set speed. It's all really controlled by computer anyway, why couldn't it brake, too? (I would have it so that it would *NOT* use the physical brakes in CC mode, those would only kick in if pedal pressure is applied.

 

[Daniel]

Hrm. Now that I'm thinking about it, you would think Toyota could have designed a more intelligent CC for the Prius. One that may be a little less aggressive on the acceleration, that doesn't automatically kick off if it can't hold the speed, etc. Another feature I'd like is for it to use regen /engine braking to SLOW the car down automatically when going downhill to maintain your CC set speed. It's all really controlled by computer anyway, why couldn't it brake, too? (I would have it so that it would *NOT* use the physical brakes in CC mode, those would only kick in if pedal pressure is applied.

Obviously, they chose to have the CC hold the car as rock-steady as possible. They could have made it softer, and then folks would complain it did not hold its speed.

But it does use regen on the downhills. One of my two road trips took me through some hilly country, and I saw regen on the downhills.

Maybe what we should ask for is a slider control for CC firmness, running from very solid to very soft. But even then, it would not be able to anticipate hills or judge the flow of traffic to decide how much speed variation is safe.

 

[patrickg]

ehurtley and Daniel, excellent thinking! Why not a variable slope on the "aggression", user adjustable. As regards CC, it doesn't anticipate a hill or its duration but it could measure the slope and get some info ahead of waiting for speed to drop as a secondary measure of steepness. Wonder what surplus TAR goes for? :wink:

I haven't checked this specifically but I don't recall the regen being used by the CC. There is regen everytime you take your foot off the gas and the car is going fast enough to produce regen. By design, the regen braking is biased on to a level that approximates the compressive breaking of a "regular" car. This way the car doesn't excessively freewheel and feel funny to folks hadituated to compressive braking in "normal" gas cars. We avoid this and "freewheel" by using the least pressure on the gas pedal that will prevent the regen from activating to its preset level. Coasting is more efficient in some situations that slowing with regen and then using the power to go again when you could have coasted there in the first place. (There are losses in getting the KE into the batt and back.)

What ehurtley was asking is what I thought of also.. Why not have the CC use regen as required to control downhill speed? Mine doesn't.

They didn't miss many "tricks" but this may be one.

Have a great day guys.

Pat

 

[ehurtley]

But it does use regen on the downhills. One of my two road trips took me through some hilly country, and I saw regen on the downhills.

Yeah, it uses regen when 'coasting' at any time. But it doesn't use it more aggressively in CC to hold your speed down. (i.e. on a big hill, I still accelerate to faster than I wanted to go, even with CC on.) I want CC to engage regen braking MORE so that I don't exceed my CC speed. I don't want to have to ride the brakes, or shift into 'B' to avoid speeding down a couple of hills. (There's one freeway hill near me where there is almost always a police car waiting at the bottom to catch people who let their car accelerate down.)

 

[CaPriusLover]

I have to agree that more aggresive regin would be good. Where I live, we have the "Sunol Grade" which will easily take you from 65 at the top to 78 at the bottom. This happens in CC. It seems that when you get to a certain speed OVER where you set your CC, the car should not be allowed to accelerate any further via the downward slope of the road.

The good part of the grade is that my battery is always charged (in the green) at the bottom.

Yesterday and today I drove to work without using CC in the hilly areas. I actually did see a slight improvement in MPG. It was only about .2 MPG, but I think the real test will be to compare my MPG over my next couple of fillups.

 

[Daniel]

Interesting. I guess I didn't have steep enough hills, because my car always stayed within about 1 mph of set speed, and I saw regen on the flow chart. This was rolling country between Sioux Falls and Albert Lea, along the southern edge of MN. Nothing really long or steep.

 

[arizonakim]

CC & Speed Limit

I love using cruise control, even around town in light traffic I will use it
(45MPH roads) because sometimes I go a little faster than I realize and it helps prevent possible tickets.
I generally drive the speed limit or up to 5miles over and while I will admit there are some roads in some places I think have "crazy" speed limits for the conditions, I rather like the wide open freeways out here in AZ with a sane 75mph speed limit.
I grew up with CT's 55mph speed limit and sometimes that just feels like you are not moving.
espec. when no one else is doing 55.....
sorry Daniel, I'm all for safety and I drive very defensively but I have to disagree with you on this one. Some roads you can safely drive 70 to 75,
and while I also like to conserve fuel when you have a couple of thousand miles to drive you don't want it to take 36 hours instead of 28
:lol: