why is Prius rated higher in a city and lower on a highway?

Re: why is Prius rated higher in a city and lower on a highw

I agree with you. I think the whole EPA mileage is done by a MPH study, and not taking into consideration the stop and go. If my city driving consisted of no lights and all I had to do is go 35 mph, I think I could get the 60 MPG. Its the lights, stopsigns and traffic that always brings my mpg averages down.

 

We have noticed the same thing. My wife does a lot of short runs that would lower any cars mileage, but I see no way that in our town we would ever get more than 46-50mpg. She discovered on her last tank that driving 60 mph on the big slab has her averaging 51 on this tank. 200 of the 325 miles were highway. There was also less load on the ac. Now, if I can just get the seat to fit me better, I could get to like the Prius.

Doug

 

Re: why is Prius rated higher in a city and lower on a highw

The Prius gets better city than highway mileage because it uses regeneration to charge its battery. As you coast and brake (just about any time your foot is off the accelerator), the wheels turn an electric motor to recharge the battery. Then when you're driving at slow speeds, the battery turns that motor the other way to turn the wheels. That results in infinite mpg, since you're not using any gas. Even when the gas engine kicks in at higher speeds or higher acceleration, the battery can assist the engine, which results in less gas use.

As you can tell, there's more coasting and braking in the city than on the highway, therefore more regeneration, therefore less gas use.

Of course, if you don't drive gently, you can still get the same bad mileage in the city that you did before (well, maybe not quite as bad). Lead feet drain gas in any car.

 

Re: why is Prius rated higher in a city and lower on a highw

We've had our computer on wheels for 10 months now and have experienced exactly the reverse..... we consistently get better fuel economy on the highway than driving in our local town.
I cannot explain it .Our highway driving at max allowed 90kph (55.9mph) includes somewhat mountainous terrain where we use cruise whenever possible but not in town where our local driving is 50kph.(31mph).
Total 15 fillups averages @ 9096 Km/5653.2m:
5.55L/100Km
51.75 mpg (Can)
43.08 mpg (US)

 

Re: why is Prius rated higher in a city and lower on a highw

Hi Magieddd,

I am not too sure about the EPA testing, but there is a good physics reason why a hybrid car could have better fuel economy in the city driving, than in highway driving. Its air.

The power to push the air out of the way is some constant times the speed to the cube power. At 60 mph it takes 8 times more horsepower to overcome air drag then at 30 mph. Energy (power times time) expended to push the car through the air is not recoverable. It heats the air and the car body. So the faster one goes, the more fuel is burned for any given distance. And that energy is now gone into the second law of thermodynamics!

At city speeds though, the main loss is rolling resistance. Rolling resistance gets less and less the faster you go. Anybody who has had to push a car from a stop knows this is true! Once the car is up to 30 mph or so, the energy lost by rolling resistance is quite small, and most of the energy in the moving car is associated with the momentum of the car. So, when one slows down with the regenerative brakes in a hybrid car, that momemtum energy is mostly recovered.

With good aerodynamic design and a small cross section area, like the Prius has, the constant for the air drag is so small, that air drag does not become large with respect to the rolling resistance to well out in speed. I remember back in the 70's that tests on the DOHC Cosworth Chevy Vega by a car company engineer showed it got better gas mileage at 65 MPH than at 55. The engineer was more interested in showing that for every rationalization behind any law, there is always an exception. In this case it was the 55 MPH law. The DOHC engine had lower engine pumping losses, which helped to push up the speed when the mileage got went from better to worse. This does not apply to Prius though. The Vega is even smaller in cross-section that the Prius, I believe. The Vega engine was also bigger, and the greater load from the 65 MPH air drag made for a load on the engine that was closer to its peak efficiency for the gear ratio. The PRIUS CVT should match the engine for optimum efficiency at almost any civilian car speed.

I do not own a Prius (yet!), or have the real world experience. But I sat through a lecture on hybrids in an engineering class, the day before Thanksgiving break back in the late 70's in a linear circuits class. That is when all the prof's would roll out their pet project (research) lectures. Because they did not expect the much of a class to be there!

I noticed the reverse fuel economy EPA specs right off in 2000. This is a clear sign that Toyota got it right. Ideally, a hybrid should be able to use less gas in city driving than on a highway. Hopefully the regenerative braking will get better and better and the real world experience will match the EPA spec qualitatively someday! The real world experience being backwards from the EPA spec might be due to higher tire rolling resistance in the production cars (can't trust those purchasing department guys!) , or that the air drag is just really really low on this car!


Bye....

 

I think it has to do with how the EPA figures are being derived. We know that the MPG have been adjusted down 10% and 22% for City and Hwy respectively. If you look at the unadjusted figures, you will be surprised that they are actually not much difference.

For NHW11 (01~03):
City MPG (Adjusted) = 52
City MPG (Unadjusted) = 52/0.9 = 57.8

Hwy MPG (Adjusted) = 45
Hwy MPG (Unadjusted) = 45/0.78 = 57.7

For NHW20 (04~05):
City MPG (Adjusted) = 60
City MPG (Unadjusted) = 60/0.9 = 66.7

Hwy MPG (Adjusted) = 51
Hwy MPG (Unadjusted) = 51/0.78 = 65.4

Vincent