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What will be the MPG when driving on the highway at 70 mph?

  • Less than 30 mpg

    Votes: 7 4.0%
  • 30 mpg - 35 mpg

    Votes: 29 16.8%
  • 36 mpg - 40 mpg

    Votes: 75 43.4%
  • 41 mpg - 45 mpg

    Votes: 42 24.3%
  • 46 mpg - 50 mpg

    Votes: 9 5.2%
  • More than 50 mpg

    Votes: 11 6.4%

Fix: What will be the MPG when driving on the highway at 70 mph?

41K views 71 replies 38 participants last post by  thetoad 
#1 ·
Don't just vote, stake your claim and post it. As the poll creator, I don't want to sway the poll so I'm going to post mine later (it's the same that I posted many times before).
 
#3 ·
Of course its a WAG, I'm staking at 41-45 mpg

Does anyone know what a Prius gets at 70mph constant on level ground?.. hint it wont be the EPA hwy mileage..

My electrical drivetrain efficiency guess, assuming each of the four components gets the same efficiency:

Best case: 98% motor/inverter/GCU/generator = total 92%

Average case: 95% = total 81%

Bad: 90% = total 65%

Really bad: 85% = total 52%

I would say a total electric drivetrain efficiency around 80% at the sweet spot is reasonable.

My second prediction... hwy and city mileage will be the same in the charge sustaining mode.
 
#4 ·
...My second prediction... hwy and city mileage will be the same in the charge sustaining mode...
I think just the opposite. I believe the more times the car is stopped and restarted the more consumption. Electric motors, especially with no transmission, use more energy to start than to keep rolling. There's a price to pay for that maximum torque at 0+ revolutions. Brake regen won't be a factor because it's not efficient enough.
 
#7 ·
I used to get 53 mpg on the highway in my Gen 2 Prius driving LA to San Diego and back (about 4 hours highway, total), and that includes gentle hills (200 feet ? 400 feet ?). I would average 70 mph, but sometimes slow down uphill (with my cruise control, click, click, click down to maybe 65 mph), and then going downhill, speed up to 75 mph (and still get 99 mpg on the meter). Traffic allowing.

The Chevy Volt is said to be slightly more aerodynamic than that Prius (Cd of 0.24, the Prius is 0.25), and I assume they will put low friction tires on those aluminum forged wheels. I'm a bit concerned about the weight of the Volt, and I'm not sure how the series-hybrid arrangement will work out (71 hp gas engine plus big battery buffer; a lot depends on how GM tweaks the parameters. As I've posted before, GM could give a choice of 'Performance' vs. 'High MPG').

Since
(drag force is proportional to velocity squared), driving at 70 mph is 36% more drag than driving at 60 mph.

Taking everything into consideration, I say 50.7 mpg at 70 mph. But that's with slight overinflation of the tires, which I do on the Prius. And note that the EPA highway tests include the gasoline needed to reach highway cruising speed - here, it is just the MPG of cruising at 70 mph on a relatively flat highway, that's why I vote for 'more than 50 mpg'.

BTW, the old EPA mpg tests had these two schedules:

Who drove 50 mph or 55 mph on the highway ? Exactly.

The new EPA measurements, started in 2008, add tests at High Speed highway, with Air Conditioning, and Cold Temperature driving.
http://www.fueleconomy.gov/FEG/fe_test_schedules.shtml

But with plug-in hybrids, the test will be different, again.
 
#10 ·
Sorry, folks, I posted two exact threads and they are both alive. Please post your comments on this thread (The other one does not have a working poll).

I will re-post some of the comments made by other posters:


Tom wrote:

See old thread for EPA profile AER Estimates: http://gm-volt.com/forum/showthread.php?t=719
Assume Driving Pattern/Profile: Cruise at constant speed, but Time for start, stop, and regen breaking, Timess, is every 15 minutes.
State of Charge Usage (battery consumption) is SOCgen. SOCgen is 50% for recharge. 320V HV battery idle power is Po. The Traction Inverter x motor Efficiency =TInvE, HV Power Electronics at Idle Efficiency = IPEE, and Gear Power Efficiency = GPE are: 90%, 95%, and 97%, respectively. Brake Regen efficiency of kinetic energy is 69% @ deacceleration = 0.315g. For SOCgen = 50% at 70 mph, range is 36miles.

Detailed assumptions: http://www.leapcad.com/Transportatio...Simulation.pdf
Below graph assumes auxiliary power of 50 and 100W.
Tom
 
#11 ·
Re-Post:


hermperez wrote:

what the heck?.. my post disappeared.

I stake the 40-45 mpg point in the charge sustaining mode.. I further make a prediction that the CS mpg in both city and hwy epa cycle will be the same.

I went ahead and rambled on about electric drivetrain efficiency and finally settled on around 80%.

I also asked, does anyone know what the Prius does at 70mph steady?.. please pipe up if you are an owner. Hint, it will not be what the epa hwy cycle says it does.

Oops I found it, Geronimo gave us a 70mph mileage on his Prius:

"I used to get 53 mpg on the highway in my Gen 2 Prius driving LA to San Diego and back (about 4 hours highway, total "
 
#13 ·
re-post:


hermperez wrote:


Originally Posted by dagwood55
The Prius can get 53 because it is very aerodynamic, it is relatively light (3050lbs) and the power transmission is efficient. The Volt is behind in each of those three criteria.
Weight I will give you, the Volt is about 700lbs heavier and that should account for a couple of mpg points. The Volt is as aerodynamic as a Prius, perhaps better.

The Prius does not have a transmission in the traditional sense, it has no clutches or torque converters of any sort. Essentially the engine is permanently in the highest gear for hwy travel and never changes from there.. this is the advantage of the Prius at hwy speeds, it is almost bypassing the electrical losses, but not quite.. the main motor will be spinning at high rpms but not drawing much power while the engine drives the wheels.. but still there are losses from all the gears spinning around.. those friction losses will blunt the Prius advantage somewhat. Usually the more powerful motor will be more efficient than the smaller motor, and the Volt's motor is twice the power of the Prius main motor.
 
#14 ·
texas: "Usually the more powerful motor will be more efficient than the smaller motor, and the Volt's motor is twice the power of the Prius main motor."

Strangely enough, Ferraris, which have very big motors, get crappy mileage. The engine should be sized to the application and, it seems to me, if big "enough", a smaller engine does better.

In any event, the Prius has a 1.8L engine of maybe 90hp and the Volt has a 1.4L engine of maybe 70hp.

texas: "The Volt is as aerodynamic as a Prius, perhaps better."

Betcha it ain't. GM compromised the shape to make it look more conventional and less Prius-like for marketing reasons.

And, the higher the speed the more effect drag is going to have. Aerodynamics will be very important at 70mph.
 
#17 · (Edited)
48.45 Mpg

1 Gallon of Gas = 36.419 kWh

http://tonto.eia.doe.gov/kids/energy.cfm?page=about_energy_conversion_calculator-basics#mogascalc

At a constant 70 MPH, Tom’s Simulation (which is best I’ve seen without actually having the proprietary information) predicts the Volt can travel about 28 miles on 8 kWh, so that equates to an electric efficiency of 3.5 miles/kWh.

http://gm-volt.com/forum/showthread.php?t=581&highlight=rooster

Thus if the ICE/Genset was 100% efficient, the Volt could theoretically travel 127.5 miles on a gallon of gas at a constant 70 MPH, as predicted by Tom's simulation.

Since the engine is not coupled to the wheels mechanically, it can always operate in the "efficiency sweet spot". (GM has hinted this is what they intend to do) So I’m going to have to take a WAG on the engine & genset efficiency -- I think the volt’s ICE/GENSET will be about 38% efficient during highway cruise under constant RPM.

Thus I predict the Volt’s MPG at a constant 70 MPH will be 48.45 MPG
 
#24 ·
At a constant 70 MPH, Tom’s Simulation (which is best I’ve seen without actually having the proprietary information) predicts the Volt can travel about 28 miles on 8 kWh, so that equates to an electric efficiency of 3.5 miles/kWh.
That would disappoint a lot of GM engineers, who have been working for years so that the car can get 40 miles on 8 kWh (half the 16 kWh li-ion battery).
Do you think GM engineers expect people to commute at 35 to 45 mph, optimum gas mileage speeds ? That would dampen the enthusiasm for the Volt somewhat...
 
#18 ·
I'm going with 45 to 50 mpg. Great tires, great aerodynamics and a optimally tuned engine that doesn't have to be able to work at lots of different RPMs should get it there.
 
#19 ·
Dagwood, a Ferrari has an engine, a Volt has a Motor .. what I was saying is that USUALLY the biggest electric motor is more efficient, in comparing a Volt and a Prius.. but it is a guess regarding which one is better.

Both Prius and Volt use a differential, so that should be a wash.
 
#20 ·
45 +/- 3

The Volt will be slightly less efficient due to the generator/motor inefficiency not being completely compensated for by the tighter control to peak efficiency operating conditions (less power demand) for the engine. Also, I'm guessing GM may tune more for performance than efficiency. A few Prius owners I know say they get over 50 MPG at 70 MPH. If you give the Volt a 10% hit on efficiency, then you get to about 45 MPG, then you need some wiggle room for unknown factors such as GM's optimization goals.

Clearly Texas hasn't voted yet, if I remember correctly his prediction was <30 MPG at 70 MPH from a previous post.
 
#21 · (Edited)
Clearly Texas hasn't voted yet, if I remember correctly his prediction was <30 MPG at 70 MPH from a previous post.


From "Chevy Volt To Get 230 MPG City Fuel Economy Rating " Post #10:

"Here is my prediction:

The Volt will get around 30 mpg or LESS when cruising at 70 mph on the highway. When the Prius is right next to it the Volt will get less mpg."


pdt, clearly you are wrong again. I did vote. I said it would be Thirty or less. Thus, I selected 30 - 35. pdt, do you ever get anything correct? Ever look up supporting information? Have no pride in your easy-to-disprove statements?
 
#23 ·
50+

I'm in the 20% that went for 50+. Of course, in a sense I'm cheating, as I still think in Imperial gallons, which are 20% larger that US gallons. :) Despite that, I see no reason why the Volt shouldn't get surprisingly high mileage. It's a small engine running at the most efficient ranges, with the battery smoothing out the load versus the demand.

It's surprising what even a standard engine can do under optimal conditions. My Mazda 6 station wagon gets 7.8 l/100km (about 32 mi/us gallon) on the New York State Thruway, which is about 3% better than the official rating. That's at 70 mph on cruise. The Volt is far more aerodynamic, and has an engine with less than half the displacement and peak power. Neither car will require their peak engine power at 70 mph, so the smaller engine should be much closer to its optimal efficiency.

I'm looking forward to a pleasant surprise when GM announces the charge-sustaining rating.
 
#25 · (Edited)
The Volt gets 40 miles range in either the EPA hwy or city cycle, which happens to average 48mph speeds.. that is a big stretch to 70mph steady on a hwy. There is no EPA 70-Hwy cycle used to compare cars to each other.

http://gm-volt.com/forum/showthread.php?t=719

using the Tesla power vs speed curves is the best hard data we have, I think they fit the Volt pretty closely... about all that we really know is that the Volt needs 50kwh to do 100mph and the 40 mile number.

http://www.teslamotors.com/blog4/

The Tesla data says 200wh/m for 48mph speed, 500wh/mile for 100mph and 300wh/mile for 70mph.. that works out to a 27 mile range.
 
#29 ·
The Volt gets 40 miles range in either the EPA hwy or city cycle, which happens to average 48mph speeds.. that is a big stretch to 70mph steady on a hwy. There is no EPA 70-Hwy cycle used to compare cars to each other.

http://gm-volt.com/forum/showthread.php?t=719
Your cited thread uses the wrong EPA test for highway driving; this test was changed in 2008 to be more realistic. Tom says Hwy max speed is 60 mph, mean is 48.2 mph. This was true in 2007 and earlier (well, 48.3 mph). But the change in 2008 specifically added Higher speeds; harder acceleration & braking, and they changed the max speed to 80 mph.
http://www.fueleconomy.gov/FEG/fe_test_schedules.shtml
Click on High Speed and Detailed Comparison.

I'm sure GM realizes the test has changed.
Anyway, there is sure to be auto journalists test driving these Volts next Summer/Autumn, and they will give detailed reports of real-world performance before I buy one :)
 
#28 ·
To reference Geronimo's previous post:
The new EPA measurements, started in 2008, add tests at High Speed highway, with Air Conditioning, and Cold Temperature driving.
http://www.fueleconomy.gov/FEG/fe_test_schedules.shtml

But with plug-in hybrids, the test will be different, again.
At this point, my vote is the lone 36-40mpg entry, based on a dash of pessimism and the fact that I never drive without the environmental system operating in some mode.
 
#36 · (Edited)
My gen2 Prius, on dead level highways running around 70mph really only gets around 46 MPG. And I would say that the Volt should be able to equal this number or slightly better it (lower Cd and special tires plus other).

On the other hand if you believe Lauckner and the Volt will really go 61.7 miles on 8kw-hr in the highway cycle (avg speed=48 MPH) then that is equivalent to 130 whr/mi=7.7mi/kwhr. Assuming an Atkinson Miller ICE eta=37%and a combined generator/inverter/motor efficiency of 92% and 33.4 Kw-hr/gal for gas, I'm at 87.6 MPG for the Hwy driving cycle. However this is for an average speed of 48MPH.

If you ratio by velocity squared the correction is .47 but I don't think it's quite that strong a function esp based on the Tesla numbers that show a correction of 200/300= .66

So at a .47 correction Lauckners numbers become 41 MPG and using .66 correction 57.8 MPG. Using a number in the middle=.57 Lauckners numbers equate to 50.

I guess I'm going to be optimistic (although my engineering brain says not to) and put my vote at 50MPG or better.
 
#37 ·
My gen2 Prius, on dead level highways running around 70mph really only gets around 46 MPG.



George, Thanks for that number. Can you give us the details of how this number was calculated? There are a few other Prius owners and if we can get a couple of people agreeing on this, we will have a good base of comparison.

1) Did you use real calculated numbers based on fill-ups or just use the Prius meter?

2) Did you do long runs at 70 mph or just short runs?

3) Did you run the test multiple times or just once?

Thanks!
 
#38 ·
Anyone catch this from the Q&A a few days ago?

http://gm-volt.com/2009/10/20/electrical-infrastructure-and-the-chevy-volt/

When someone asked if they could drive a Volt without ever plugging it in.

4:46
Mark Duvall:
You can drive plug-in hybrids or extended range EVs (the Volt) without ever plugging them in. * You would get approximately the fuel economy of a good gasoine-only hybrid. * So better than a gasoline car, but not nearly as good as it could be---so plug it in!
From that, I'm guessing about 40 mpg.
 
#40 · (Edited)
Anyone catch this from the Q&A a few days ago?
Mark Duvall:
You can drive plug-in hybrids or extended range EVs (the Volt) without ever plugging them in. * You would get approximately the fuel economy of a good gasoine-only hybrid. * So better than a gasoline car, but not nearly as good as it could be---so plug it in!
From that, I'm guessing about 40 mpg.
What I'm getting from that is that it will do the same as a Prius (46?), but the car will cost more. That is, unless he's fibbing about the Volt being as good as a good hybrid.
 
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