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Discussion Starter #1
I was cruising on the freeway today and saw a 2004 Prius driving along. I slowed down and followed it for a while noting that it was traveling at 70 MPH. Now I'm wondering if this the best criusing speed for the Prius milage-wise? Would it hurt milage by a lot if I happen to enjoy cruising at 85mph?
 
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Somewhat

Depending on terrain and wind, you probably would be getting between 38-42.
 

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If you have a regular highway commute, you might consider resetting your trip computer before that trip. Drive the trip at 50 one or two days (road conditions permitting). Then reset the trip again, and drive at 60 a couple of days. Then reset it again, and try 70.

This method will make it more difficult for you to see your tank mileage (during the duration of your testing), but it will give you a better idea of how your Prius handles the freeway at different speeds.

It also negates the necessity (and inherent inaccuracies) of having you constantly look at the MFD while driving. Just drive at your chosen speed, and record your mileage when the trip is done.

Consider doing your test with "round trips." Sometimes the trip in one direction or the other is mostly uphill, downhill, or into/with the wind. Recording your one-way direction may be misleading. After all, you want to know what's the best overall highway speed, and your Prius is not equipped with a windspeed indicator, so you cannot make mental or mathematical adjustments for wind.

On my car, I believe the best highway speed (for MPG) will be approximately 50-55 MPH. I was driving 48 yesterday on a local street, and finally got one green light (so I didn't have to stop). At a steady 48 MPH on a relatively flat grade, my MFD was consistantly showing between 68 and 98 MPG. Even the worst mileage I was getting on that mile-stretch was better than the EPA estimates for the car!

I do plan to experiment more scientifically, but probably not until after a few thousand miles on the ODO. I'd rather let the car get broken in really well first, so I don't have to do the tests over.
 

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Discussion Starter #5
I was under the impression that since the IC is on all the time anyways on Highways I if I cut the trip time in half 35min(85) vs 60min(50) I would do well by having the engine run at optimium for for less time. I'd love to scientifically test it out for myself but, I'm still waiting for a Prius to come in. (2-3 month wait, I've waited two-three weeks so far). :( I'd actually be happy with 42MPG on highway, thats about double what I'm getting now.
 

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lasrx said:
I was under the impression that since the IC is on all the time anyways on Highways I if I cut the trip time in half 35min(85) vs 60min(50) I would do well by having the engine run at optimium for for less time.
Mmm, well, that's a bit like driving home faster when you're drunk to minimize the chances of an accident. The Prius runs it's ICE so as to minimize emissions, which is not always the same thing as maximizing fuel efficiency. Also, aerodynamic drag increases roughly as the square of the speed, and at highway speeds drag is the largest single loss, so doubling your speed (halving the trip time) more than doubles the power required, and so the total energy required for the trip is larger.
 

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The ICE may be on all the time at highway speeds, but that does not mean it is consuming the same amount of fuel. Because wind resistance increases rapidly with speed, it takes a LOT more fuel to go fast. And the Prius, because it is a hybrid, and because of its Atkinson-cycle engine, is able to remain efficient at lower speeds than a conventional car. Thus those phenominal mpg numbers for city driving.

That said, you'll get where you're going sooner at 70 than at 55, and you have to decide between saving some gas (and being green!) or saving some time on your trip. Some folks are willing to spend a little more time in their car and know they are doing their small bit to help the environment. Other folks are in a hurry.

But yes, your mileage will be very much worse at 85 than at 65.
 

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This past April, my wife and kids and I made the 400-mile trip from Madison, WI, to St Peters, MO. We drive between 70 and 75 on the big highways. Our drive-down MPG was 48.5, but our return-trip MPG was 38.5.

The difference was the wind in the flats of Illinois. It took a full 10 MPG off the return trip.

Our mileage also takes a beating during the coldest months of winter. Such is life in the snow belt.

I have a green 02 Prius with WI vanity plate SULEV. See you out there!
 

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lasrx said:
I was under the impression that since the IC is on all the time anyways on Highways I if I cut the trip time in half 35min(85) vs 60min(50) I would do well by having the engine run at optimium for for less time. I'd love to scientifically test it out for myself but, I'm still waiting for a Prius to come in. (2-3 month wait, I've waited two-three weeks so far). :( I'd actually be happy with 42MPG on highway, thats about double what I'm getting now.
Just to be clear here, time is not the issue. Distance, or miles is. You are traveling a fixed distance either to minimize time, maximize fuel economy, or some of each. Driving 85 mph will net you about 40 mpg. Driving 75 will net you about 50 mpg. Driving 65 will net you about 55 mpg. No matter how long it takes to get somewhere, you are using gas. The faster you go, the more you will consume, in general. Increasing your travel time reduces wind resistance, which increases fuel economy.
 

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BTW, any car will get significantly worse mileage at 85 than at 70. It's simply a question of wind resistance. It takes a lot more energy to overcome the exponential increase in wind resistance with speed.

That's why, under President Carter's lead, we passed a national 55 mph speed limit to conserve gas (and, incidentally, to reduce the number and severity of traffic accidents.) Carter pointed out that we waste more energy than we import.

But gas was cheap, and Americans are always in a hurry, and subsequent leaders thought that belligerant posturing would do more for American security than energy self-sufficiency would do, and we are now back to break-neck speed limits.

But you'll still get there on less gas at 55 than at 70, and 85 will cost you a lot more gas than 70. Of course, it's your choice. It's a free country. Depending on the speed limit.
 

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I drive 100 miles a day on freeway mostly. I typically speed around 73 mph and get 50 mpg average. If I don't pay attention on how I drive, I get about 42 mpg.
 

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My daily commute is 160 miles/workday, and about 80% of it is highway driving. For the stretches of highway that have a 60mph speed limit, I usually go 63 mph and get between 51-57mpg. For the 70mph speed limit, I usually drive 72mph, which gets me between 49-52mpg.

Once I actually move (i.e. buy a house between my job and where I'm commuting from now), I plan to try doing no more than 55mph to see how much of an increase it is.
 

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Discussion Starter #14
50mpg @ 70 is great, does it require any special techniques? Or will just criuse control be enough?
 

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I have not made the comparison myself, but several people have reported that cruise control on the highway will get you better mileage than you can do manually.

What I can say from experience, is that the cruise control on the Prius is exceptionally steady. There's about a 1 mph overshoot when changing the setting, very briefly (a matter of some seconds) and then it pretty much stays spot-on.

A conventional car might do better with steady accelerator pressure on hills (letting it speed up on downhills and slow down on uphills) but because the HSD can draw extra power from the battery on the uphills and recapture it on the downhills, it can maintain a steady speed on hills with little loss of overall mileage.

Whether you ask about highway or city driving, there is controversy: some folks have special techniques; others say, "Just drive it!"
 

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I got the car in Nov '03 and drive around 85 miles round trip a day. In the cold months I got 49-50 mpg. I got few 55.5 mpg per tank but often get 53 mpg. I do not know about the technique but I believe when you press the brake slightly, the regeneration would slow down the car not the brake; but if you brake hard, then the brake pad will come in and energy is waste here.
 
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