this is my first thread here in the PriusChat forums. I bought my Prius c 2 a couple of weeks ago and have put about 700 miles on it since, taking mostly longer trips. I'm loving it. The trips have mostly involved lower-speed highway driving (55-low 60's mph) and some freeway driving, where I still try to keep the speed down. Between 35 and 45 mph, it seems relatively easy to get low-60's mpg on quiet backroads. However, the reason for my post here is I wanted to point out the main pattern I'm seeing with this car- that drag and wind are huge factors in the mpg it gets. When the wind picks up, either from an angle or ahead, straight on, the engine has to work extra hard even to keep 50-55 mph up and the mpg gets slammed down into the 40's. HOWEVER- when the wind is behind the car, or when there is a large truck or 2-3 cars in caravan ahead to draft off of, this car gets outstanding mpg. Drafting at a safe distance often bumps the fuel efficiency up on this car 7 to 10 mpg, as far as I can tell (keep in mind I'm not doing any kind of technical testing- this is just my observation by the digital trip mpg indicator and 5-min consumption). Today, heading east and with some reasonable drafting, I got 63.5 mpg (by the digital indicator) over a 90-95 mile trip. On the return trip west, same route and at roughly the same speeds, but heading more into the wind, I only got about 54 mpg, even with drafting. My point is that, while speed is a primary factor to consider when trying to get the best fuel efficiency out of this car, I think that the main reason speed matters so much is the increased drag on the car at higher speeds, and not so much decreased engine efficiency (though I'm sure that decreased engine efficiency factors in significantly, too). Focusing on shielding this car from being beaten by the air in front of it, no matter the speed- SAFELY- is a great way to get significantly increased mpg in it. It perhaps matters more than the exact speed the car is traveling. When I'm out on the highway, I'm looking for a big vehicle to get behind at a safe distance and follow at their lower speed. This is the strategy that has given me the greatest success in increasing this car's fuel efficiency on long trips.
I think that you are right. I have a 2012 liftback, and live in the S.F. Bay Area, and when I do my 35 mile commute on a windy day, my mpg drops from ~ 50 mpg to between 40 ~ 45 mpg. It doesn't matter whether it is a head wind or a side/cross wind, my mpg drops. At least this has been my experience over my first 1500 miles.
Yeah, cross-wind is just as frustrating- it destroys drafting, too. One of my favorites is when you get a concrete separator on a two-lane highway that shields from the wind- a caravan of cars can create an awesome wind-tunnel that makes mpg skyrocket. BTW, if you're talking about the wind on 80, going over the hill to Vallejo- I know exactly what you're talking about!
You're right. The Prius family of hybrids are so economical that small mpg killers such as wind resistance, wet roads, slightly off tyre pressures make a noticeable difference. 5% loss on 30 mpg isn't really that noticeable but 5% loss on 60 mpg is.
As a rule of thumb: E.g., 55 mph with a 10 mph headwind = 65 mph with no wind or... 75 mph with a 10 mph tailwind = 65 mph with no wind
Was there a elevation difference? Even a small difference (600 ft) can also have a huge impact on MPG when you compare return trip.
http://priuschat.com/forums/other-cars/95536-car-driver-mileage-no-its-your-gallonage-really-counts.html Consumer Reports tests speed vs MPG & comes to false conclusion about Honda Insight 2 - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com From U.S. GAO - Energy Efficiency: Potential Fuel Savings Generated by a National Speed Limit Would Be Influenced by Many Other Factors and many other sources:
congrats on your new 2012 Prius c. You will also find that for larger vehicles just driving slightly behind them from the side on a parallel lane also can increase your fuel efficiency (side drafting). However, there is also another fuel efficent way of driving on the superhighway beyond drafting in the slipstream of larger vehicles. It's called DWL. At high speeds, driving with load (DWL) can be a very fuel efficient driving method for the Prius (HB/v/c). DWL attempts to conserve gravitational energy by speeding going downhills --building up momentum-- and then slowing down going uphill -- giving up momentum--. DWL is more important FE driving technique for vehicles like the Prius with a low Hp power to curb weight ratio (and less important to vehicle with hi HP power to curb weight ratio like a Porshe 911). Vehicles with hi curb weight can stores inertia energy generated from a downhill push to be used later for an uphill climb. It is also more fuel efficient to use the ICE to gain speed going downhill than it is going uphill so that why you accelerated going downhill in DWL. It is also more fuel efficient to avoid running the ICE hard and allow the Prius to lose speed on the uphill so that why you decelerate going uphill in DWL. Driving continuously at highway speed, Eco mode is useless, so set the Prius to Normal mode when you are doing DWL. If you have the ScangaugeII the most fuel efficient burn rate for a ICE on a 2010 Prius is from .80 gph to 1.10 gph (but if you dont have a ScangaugeII then look at your HSI display. Press your accelerator so that the HSI Display is alittle bit pass the right of the center line of the HSI bar) as it goes uphill. Newton states anything in motion tends to stay in motion unless acted by some force. So its best to accelerate from the top of hill to your desire top speed and then glide until near the bottom of the downhill grade when you need to accelerate to gain extra speed before the uphill climb. Before the uphill climb, you accelerate the Prius about 10 mph more than your desired top speed. Once you've gain the extra speed for the uphill climb, you pull back on the accelerator dropping the ICE to its most efficient burn rate which is slightly right of the center of the HSI. As your Prius climbs uphill -- it will lose speed the extra speed you just pumped into and maybe more until it reaches the top of the hill. At the top of the hill let go of the accelerator and glide letting the Prius' momentum push you pass the apex of the top of the hill until the Prius is pointing downhill again. Push the accelerator down to return to your desire top speed and repeat the process again..
Driving slightly behind and to the side in a parallel lane of another vehicle is fine as long as you're not in their blind spot. Many drivers have lost the ability to turn their head when changing lanes.
There are hills, but there's not really a net difference in elevation due to a portion on the far westward end of the route (near my home) that is basically at sea level. I.E., started more or less at sea level, drove east for 95 miles at 63.5 mpg (but with the wind) to roughly the same elevation (net), then against the wind on the way back. It is possible that taking the hills is easier going east, but the truth is that the wind is just really noticeable when going west. It blows the car around and forces a lot more throttle.
To Cwerdna: Yeah, drag increasing exponentially with relative speed to the air is pretty much dead-on what I'm talking about. It can really be seen in the performance of this car. The car gets such great mpg at 35-45 because drag isn't yet much of an effect on it at that speed. Above 55 there is a huge difference without finding a way to mitigate the drag, but, once you do find a way to mitigate it that high, the speed itself doesn't matter as much with regard to fuel efficiency. I.E., above 55, go the speed you need to go to keep the air from beating the car, and don't worry as much about throttle efficiency at speed. Mitigating the air overshadows differences in engine efficiency- your engine's at 50 mph while your car is going 60 mph. Thanks!
The Prius c is much lighter than it's cousins, too- not much inertia to it. Probably part of the reason, with exterior design, why drag has a large effect on it.
Ok, that was an incomplete statement but I think you got the jist, lol. It is less effective on longer grades since the momentum is lost and the engine has to be engaged to maintain speed.
I drive about 70 miles a day to work and I notice major differences on windy or cold days as well. My drive is primarily 55 to 62 mph with many small hills and while on the highway my average mpg readout is often low to mid 40's. And I notice it takes a long time to start creeping toward mid-40's. It does start creeping higher when I get into the little town I work in. Before i get on the 55 mph highway, I have a short 5 mile run on a freeway which has a speed limit of 65 and i drive about 69 mph. In that short strech, the car is only showing 38 to 40 mpg's at best probably cause the car's not warmed up and the acceleration of getting on the on-ramp. Although I've had days of getting 59 mpg's when pulling in my work parking lot, I've had other days as low as 45 so there's definitely alot of flucuation due to wind and stuff.
I noticed this as well as well as I think elevation is a factor as well for low MPG. I live in Colorado and most of the time the wind is blowing through the mountains. I'll give you an example I drove from Denver to Colorado Springs (60 miles) and it was a bit windy, also the Elevation in this trip goes from (5,280 ft at Denver) to (7,300 ft at Monument Pass) and the back down to 6,200 ft at Colorado Springs. That factor + Wind gave me an average MPG of 42.8 driving there. Now on the way back (headed North) less wind.. I got near 50 MPG. I do believe that wind does have a bit to do with the MPG, as i am sure it is with any car. I am interested to know though how higher elevations affect the car's performance and MPG. I was thinking of taking it up to Mount Evans which is over 14,000 ft from Denver which is at 5,280 ft. Again this is a gradual climb at 2-5% grade, but i'd imagine that has something to do with as well as the wind factor.
I would agree with most the factors above. The Prius C higway mps's may focus attention more and more on Cd, ideal aero-shapes, and experimental A-B-A testing for most driving techniques. This data is not new. Wind tunnel data for different concept and "actual assembly line" cars and truck is widely available. Toyota spends considerable expensive wind tunnel each year to improve the mpg of each of the Prius generations and Prius family vehicles. The Prius Hatch Back is reasonable close to a well-tested "aero-template (3-D shape). The Prius C is less so. The roof-line and the trunacated rear "bluff" rear profile is not helpful. It will generate a higher Cd. That is why those who are drafting with the Prius C are seeing dramatic mpg increases. For those interested in this 72 year-old scientific data base can find Ecomodder.com. Phil Knox thread searches is a good place to start. The Prius Hatchback,V, and C already use many of the sutble wind tunnel verified aero-mods. All three Prius models have more aero potential. After market "kits" are already being produced and for sale in Japan.
Sorry, but no, the energy required to maintain speed goes as the fourth power of the speed, not exponentially. Energy is lost in, the inefficiency of the car, the brake pads, and to the air. If you aren't braking, air resistance is where you are going to see the most difference in mpg, in any car. The Prius is just more noticeable than other cars.
The way to increase your mpgs is to just slow down. The 55mph highway is about 2 miles from my house and then I have to drive 24 miles on the expressway and then 1 miles off the expressway to college. I used to get around 60mpgs by the time I got to college and the same on the way back home, but then after hyper mailing and optimizing my drive, I consistently reach 70+ mpg with 1 time I reach 76 mpg on the highway. How I do this is that I accelerate very slowly in the 2 miles before I reach the expressway and try to drive mostly in EV mode after the engine warms up. This way, by the time I get to the expressway, I am getting 48 mpg. I have a large ramp up to the expressway and by the time I am at highway speed, my mpgs drop to 43mpg. I go 54 mpg and set cruise control all the way to college and then my mpgs go all the way up to 70 mpg. Also when I reach hills on the highway, I lower my speed as I get to the top of the hill to 51 mph and then bring it back up to 54 mph on the way down using my cruise control toggle arm.
