Coastdown Facts

Overview

A coastdown test is a procedure that determines metrics later used to calculate a vehicle’s fuel economy values or “MPG rating.”   MPG ratings are established using a machine called a “dynamometer.”   A dynamometer is like a treadmill for vehicles, enabling vehicles to be operated indoors on a stationary platform to simulate real-world vehicle operation.  The level of resistance on the dynamometer is adjusted based on coastdown testing for each specific vehicle model to simulate the level of resistance that the vehicle would encounter if operated on the road.   Coastdown testing is used to determine the appropriate resistance levels (or “road loads”) to use on the dynamometer for a given vehicle model.  Coastdown testing is used to measure all types of resistance encountered by a given vehicle model during real-world operation, including:

• Vehicle aerodynamic resistance, a factor affected by the vehicle’s shape, which determines how much energy the vehicle uses to push air out of the way as it moves. The more resistance, the more energy has to be expended.
• Tire rolling resistance, a factor related to tire design that determines how much energy the vehicle has to use to overcome the resistance caused by the interface between the tires and the road.
• Driveline and powertrain mechanical resistance, a factor measuring the vehicle’s drivetrain and how much energy the vehicle has to use to overcome internal friction to drive the wheels.

Coastdown Method

• A vehicle that has been properly broken in prior to the test (generally including vehicle and tire mileage, fluids and fuel, and vehicle warm-up) is driven up to a certain speed, typically around 80 MPH, after which it is put into neutral and allowed to coast until its speed drops below 9 MPH.
• Special devices in the vehicle accurately measure environmental conditions (ambient temperature, humidity and barometric pressure), performance data, and speed and distance traveled during the coastdown test.
• In order to eliminate the effect of wind speed and direction, the test is performed multiple times (a minimum of 5 runs) on a completely flat, straight and dry road in both directions of the track.  Analysis of the recorded speed and distance information provides the vehicle’s road load force.

Background

• A dynamometer is an electric motor that is connected to the vehicle and simulates standard highway and city driving cycles, enabling generation of simulated fuel economy and emission values. 
• EPA began to publish fuel economy results, after some period of just measuring emissions, as a by-product of such emissions testing.
• Manufacturers began to reduce road load in response to public demand for more fuel efficient cars.
• The coastdown test is the standard industry technique to determine vehicle road load, which coupled with dynamometer testing, enables the manufacturer and the EPA to measure fuel economy. 
• The Society of Automotive Engineers (SAE) developed a standard procedure (J2263- Dec 2008) to perform road load measurement using coast down testing.
• The Society of Automotive Engineers (SAE) developed a standard procedure (J1263- Mar 2010) to perform road load measurement and dynamometer simulation using coast down testing.

Resources

The current government-approved standard for road load measurement using onboard anemometry and coastdown testing techniques is SAE International Standard J2263, which was most recently updated in 2008.