MPG higher on the highway

[Stallion]

Why is that? You know how you see MPG ratings when you see the specs on a car, and it's (for example) 13 city, 17 highway. Technically, why is this?

 

[corvettecrazy]

on the highway you are not changing speeds as much as you do around town, this means that the engine can run at low RPMs which provide better gas milage. I don't know how you drive but around town I see 3500 RPMs a lot, which just means more gas consumption, lower MPG.

 

[DRTH VTR]

On the highway, all fuel is used to overcome resistance to forward motion. Acceleration occurs once. In town, some of the energy is wasted as brake heat, resulting in kinetic energy being converted to heat. Acceleration also occurs more often.


In the hybrid cars, town mileage is better than on the highway. Braking is done by converting knetic energy into potential energy that goes into the battery. Less is wasted heating brakes. The highway mileage can be worse in the hybrid cars because resistance to movement (mostly air resistance) is close to the square of the speed, so faster speed means more resistance. I hope this makes some sense.

 

[JohnZ]

You see two ratings (city & highway) on price stickers and in advertising because that's the law; Federal regulations require that manufacturers show the results of the "city" and "highway" tests mandated by the EPA as part of the emissions certification procedure for that particular powertrain in a given test weight class of vehicle.

Highway cruising only requires enough power to overcome air resistance and driveline and rolling friction, with acceleration at a minimum; typically, it only takes about 30 horsepower to cruise at 50-55mph, and fuel mixture is much leaner at cruise, along with advanced spark timing. The "city" test includes a lot of stop-start/slow down-accelerate in that prescribed test cycle, and it takes much more fuel to accelerate the mass of the car up to speed than it does to simply keep it moving at a steady speed.

 

[Ken]

Fuel Economy

Why is that? You know how you see MPG ratings when you see the specs on a car, and it's (for example) 13 city, 17 highway. Technically, why is this?

Q. HOW ARE FUEL ECONOMY ESTIMATES OBTAINED?

A. The fuel economy estimates are based on results of tests required by the U.S. Environmental Protection Agency (EPA). These tests are used to certify that vehicles meet the Federal emissions and fuel economy standards. Manufacturers test pre-production prototypes of the new vehicle models and submit the test results to EPA. EPA re-tests about 10% of vehicle models to confirm manufacturer's results. The vehicles are driven by a professional driver under controlled laboratory conditions, on an instrument similar to a treadmill. These procedures ensure that each vehicle is tested under identical conditions; therefore, the results can be compared with confidence.

There are two different fuel economy estimates for each vehicle in the Fuel Economy Guide, one for city driving and one for highway driving. To generate these two estimates, separate tests are used to represent typical everyday driving in a city and in a rural setting. Two kinds of engine starts are used: the cold start, which is similar to starting a car in the morning after it has been parked all night; and the hot start, similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.

The test used to determine the city fuel economy estimate simulates an 11-mile, stop-and-go trip with an average speed of 20 miles per hour (mph). The trip takes 31 minutes and has 23 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. The maximum speed is 56 mph. The engine is initially started after being parked overnight. Vehicles are tested at 68 F to 86 F ambient temperature.

The test to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile trip and averages 48 mph. The maximum speed is 60 mph. The test is run with the engine warmed up and has little idling time and no stops (except at the end of the test).

NOTE: To make the numbers in the Fuel Economy Guide more useful for consumers, EPA adjusts these laboratory test results to account for the difference between controlled laboratory conditions and actual driving on the road. The laboratory fuel economy results are adjusted downward to arrive at the estimates in the Fuel Economy Guide and on the labels seen on new cars, light trucks, and vans. The city estimate is lowered by 10% and the highway estimate by 22% from the laboratory test results. Experience has proven that these adjustments make the mileage estimates in the Fuel Economy Guide correspond more closely to the actual fuel economy realized by the average driver.

For more information concerning fuel economy questions, see: http://www.fueleconomy.gov/

 

[corvettecrazy]

I like all your physics terms but a few things need changing. (yes I am going to sound like an a-hole for the rest of my post)

On the highway, all fuel is used to overcome resistance to forward motion. Acceleration occurs once. In town, some of the energy is wasted as brake heat, resulting in kinetic energy being converted to heat. Acceleration also occurs more often.

Because acceleration is a vector you can stay at a constant speed on a highway and still accelerate by changing direction.

In the hybrid cars, town mileage is better than on the highway. Braking is done by converting knetic energy into potential energy that goes into the battery. Less is wasted heating brakes. The highway mileage can be worse in the hybrid cars because resistance to movement (mostly air resistance) is close to the square of the speed, so faster speed means more resistance. I hope this makes some sense.

Don't hybrid cars get better gasmilage around the town because they use the electric motor to accelerate from stop lights(etc.) The gas or diseal motor then starts up after the car is moving. Since it takes more engery to get an object at rest moving the electric motor is used, which prevents excess use of gas. Therefore their highway MPG is lower because while on a highway the gas motor is used not the electric one.

"air resistance" you mean drag. You must be able to balance the drag force of the car going down the highway. This force is created by the motor, which uses gas. So the slower you go the less drag force there is, which means the motor doesn't have to work as hard to balance the drag, the better MPG you get.

 

[DRTH VTR]

Because acceleration is a vector you can stay at a constant speed on a highway and still accelerate by changing direction.

True enough. I don't believe rotational acceleration and direction change takes nearly as much energy off the car as does braking, but you make your point.

Don't hybrid cars get better gasmilage around the town because they use the electric motor to accelerate from stop lights(etc.) The gas or diseal motor then starts up after the car is moving. Since it takes more engery to get an object at rest moving the electric motor is used, which prevents excess use of gas. Therefore their highway MPG is lower because while on a highway the gas motor is used not the electric one.

Also true, but where does the electric power come from? It is either converted from the rotaional energy of the engine, or by converting the forward motion of the vehicle into electricity via a generator. Both of those things eventually require a fuel input.

"air resistance" you mean drag.

I participated in a lengthy thread where people split hairs over the meaning of the word drag. I tried to avoid this by using air resistance. Perhaps rolling resistance wouuld have been better.