Interstate MPG

At freeway speeds, drag is the most significant factor, by far, drag force almost proportionate to speed squared, power to speed cubed! And headwinds add on and cross winds can give significant apparent head winds (vector add). That being said, on my trips from San Diego to Barstow and back I usually get low 40's on the way there and high 40's on the way back (~3000' elevation change), speeds varying between 70 and 80 mph. And when the winds blowing in the Cajon Pass, it drops. YMMV!

 

What speed were you trying to maintain with CC? Also, I've found that CC is pretty aggressive with its speed maintenance. I think that this is particularly true in hilly country. I used it once on a 10 mi trip to Boulder, CO and found that I did much better without CC, in terms of MPG. You might wanna try the same trip w/o CC and see how you fair.

 

Varying your speed is not necessary (nor specified by the owner's manual), and may be causing you problems with your milage. Cold doesn't help.

 

Once its warmed up, why does cold matter, unless you're able to get the ICE to turn off long enough to cool down and thus come back on sooner? I don't think I could do that at highway speeds. If you're running the AC as a heat pump that will be some load, but if the AC is off you're just heating the cab with waste heat + some energy to turn the fan.

 

Like I said, varying your speed is not necessary (nor specified by the owner's manual), and may be causing you problems with your mileage.

Also, to elaborate, cold is cold. Your engine runs longer to maintain a constant temperature. Period. At highway speeds the heat loss is greater.

 

Is this completely true? My understanding was that the Prius heater was electric, explaining why you can get instant heat in the morning without having to warm the car up. If this is the case, then running the heater at any time would draw battery power and cause engine drag from increased MG2 usage, dropping your mileage.

I do fairly little highway driving (just under 30% of my miles) and it tends to be in chunks since my daily commute doesn't involve the highway. What I've seen from the couple of highway trips I've taken, cold definately affects your highway miles. I did one 100-mile trip at 70 degrees (~52 mpg) and another at 40 degrees (~45 mpg) and drove them in similar fashion (CC at 70 mph, approximately.) Very flat driving, by the way, making cruise control very efficient. I do believe I used the heater during the colder drive, but only sparingly.

 

the electric driven A/C compressor on the Prius is NOT a heat pump its' a variable speed scroll compressor only. If you are on the hi-way and you have it on Auto the compressor runs at the speed that the humidity sensor determines that it needs to condition the air going thru the evaporator/heater assy. Also the heater in the Prius has two small electric heater elements embedded in the core that provide supplemental heat when the flow control valve has the coolant diverted to the ICE from the thermos

 

Most of you will say I am just trolling, but we have taken in 5 priuses in trade over the last 2+ years and all but one were because of dissapointing highway mileage. Mid 30's was all folks were getting.

 

Yeah, I'm disappointed (note the sarcasm) with 36 MPG at 10 degrees F and 30 (yes, thirty!) plus MPH headwind going uphill (about 600 feet elevation to 3200 elevation over 150 miles). Speeds are 65 MPH.

Same trip with LOTS better conditions (warmer, less windy) I get 49 MPG.

Stupid people. No other changes except weather. It's just so hard to get people to realize they don't drive in a lab on rollers no faster than 48 MPH.

Sheesh...wonder what their non-Prius will get...oh, wait, they'll never know or care since many cars do not show you instant and tank MPG.

 

I'd be disappointed too, if I was getting 30 mpg. Trade the car in? I'm not sure I'd do that, but I'd be much more inclined to consider it at 30 mpg.

 

The reason you don't vary the speed during the break-in period is that the revolutionary power train of the Prius almost entirely de-couples engine speed from vehicle speed, such that the engine varies speed on its own, and changing vehicle speed has minimal effect on engine speed.

At 75 mph against a 20 mph tail wind your wind speed is 95 mph. You'll take an ENORMOUS mileage hit, due to wind resistance, which is proportional to the square of the speed.

Cabin heat comes almost entirely from engine waste heat. With the cabin heater on, the engine must run more to stay hot enough to provide that heat. A conventional car has its engine running 100% of the time, so provides excess heat. The Prius gains effenciency partly by shutting off the engine when it's not needed, but running the cabin heater prevents the car from doing this as much, thereby forfeiting some of its potential efficiency gain.

I've never taken a long winter road trip, but in ND (with its nearly-constant high winds) I generally got low 40's for mpg at 75 mph. If you got mid-to-upper 30's in an Iowa winter, I'd say that could be reasonable. Slowing down will help a LOT. Turning down both the temp setting and the fan speed, and wearing a sweater, may help also. Wearing a parka and bunny boots and shutting off the heat entirely will help even more, though the engine still have to run more to keep itself warm. And this may be much less significant on the freeway, where the engine runs constantly anyway.

Seems to me I've seen below-40 mpg at 75 mph against a fierce headwind.

 

Varying your speed on a flat surface that's probably true, but with hills I think it makes a lot of sense. Why accelerate hard to maintain a certain speed on the uphill and then brake to maintain that speed on the subsequent downhill? I generally ease things off on the uphills and build up a good head of steam (within safety limits, of course) on the downhills. That extra momentum helps me crest the next hill using as little energy as possible while still maintaining a safe speed.

 

I strongly suspect that the entire issue is weather.

I am blessed to live in a very mild climate and I find that travelling 65-70 mph on highways, with occasional slower sections due to heavy traffic, I have been exceeding the EPA highway estimates, getting above 50 and as high as 52.5 for long stretches.

If only I could do something about the shorter trips to work in the morning.

 

Weather is a major issue. Headwinds at 75mph will kill the mileage. Remember this is a light vehicle so there is much less momentum than a big car so wind and high speed will have a bigger impact. Also, cold single digit temps will force the engine to work harder to maintain temperature of the engine, exhaust system and passengers.

Not sure why your mileage is that bad, probably just the speed. In the NY.NJ area I typically do about 60-65 mph and on the highway in my Classic '02 Prius get about 50 mpg. In the winter that will drop to about 46 mpg, but never in the 30's. To get in the thirties I would have to hot rod from lights and drive like a total fool - even then I am not sure it would drop that low.

 

Finman you have got it. EPA is a lab test done under standard conditions. Variables are decreased or eliminated from car to car. Shoot you could pull the body seats and just do it with the drive train they really are not necessary for the test. Given all of that if you asked me to design a test to compare one car or truck with another I would think that the EPA would be an OK way to go. Now if you want it to reflect the real world you would have to change the test entirely. You would need many cars over many hours in all kinds of weather and road conditions. Boggles the mind. Could the EPA make a few changes to better reflect real world and still decrease the variables and make it a valid test? I would say yes I for one would like the highway speeds increased. The bottom line is your mileage will vary! Trite but never the less true. People don’t get it and don’t care. Yet! The end is near!

 

All you have to do is drive the Prius like it is a plane. When you are going into a headwind your ground speed should be lower. When you have a tail wind you can go faster to make up for the time loss on another trip. Should be easy to stay over 50 mpg at least on the highway.

But even with no wind you can't expect good mpg even up to 40 mpg if you drive fast in cold weather.
Even 30-40 deg F will seriously kill your mpg if you drive over 60 mph or so. If you like to drive 80 or 90 save it for summer.

 

If I had to venture a guess, the OP was going closer to 75 than 60-75. I also suspect the OP was aggressive to make sure they sayed 75 on the uphills. NO CAR is going to do well on mileage in that. We have had low teens temps here and the lowest my mileage go was 41. Of course, I don't drive over 65.

 

That's what I've discovered here in CO. The cruise is aggressive on the uphill but then backs off on the downhill to keep the speed as close to the target as possible. The downhill part is the real bugger because you loose out on letting gravity help out. Glad to hear that you're seeing a marked improvement.