Drag/wind/air resistance seems to be the real mpg killer

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.

 

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.

 

The aforementioned DWL technique on longer grades unfortunately.

 

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.