Vehicles are placed in "inertia weight classes" based on their weight
in 500 Lb. increments.  (Too course in my judgement but that is the
way it is.)  

<http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=ccab9e991dc3344fff92610
57c8e6e08&rgn=div5&view=text&node=40:29.0.1.4.41&idno=40%5D>

(2) A vehicle may be tested in different vehicle configurations by
change of vehicle components, as specified in paragraph (b)(1) of this
section, or by testing in different inertia weight classes. Also, a
single vehicle may be tested under different test conditions, i.e.,
test weight and/or road load horsepower, to generate fuel economy data
representing various situations within a vehicle configuration. For
purposes of this part, data generated by a single vehicle tested in
various test conditions will be treated as if the data were generated
by the testing of multiple vehicles.

    Note how in this example from the CFR on how to calculate mpg
values, the inertia wieght class and the transmission are all that is
needed to determine the CAFE mpg fro a vehicle with a given engine:

Step III. Determine base level fuel economy values.

A. For all the base levels except the base level which includes 4,000
pound, manual four-speed transmission data, the base level fuel
economy is as noted in Step II since only one vehicle configuration
was tested within each of these base levels.
      
3,500 lb/M4 transmission    16.1001 mpg.
3,500 lb/A3 transmission    15.9020 mpg.
4,000 lb/A3 transmission    13.8138 mpg.
4,500 lb/A3 transmission    13.2203 mpg.
5,000 lb/A3 transmission    10.6006 mpg.

The test procedure is codified in the Code of Federal Regulations (see
above link) which reads in part:

§ 600.111-08   Test procedures.

(a) FTP testing procedures . The test procedures to be followed for
conducting the FTP test are those prescribed in §§86.127 through
86.138 of this chapter, as applicable, except as provided for in
paragraph (b)(5) of this section. (The evaporative loss portion of the
test procedure may be omitted unless specifically required by the
Administrator.)

(b) Highway fuel economy testing procedures . (1) The Highway Fuel
Economy Dynamometer Procedure (HFET) consists of preconditioning
highway driving sequence and a measured highway driving sequence.

(2) The HFET is designated to simulate non-metropolitan driving with
an average speed of 48.6 mph and a maximum speed of 60 mph. The cycle
is 10.2 miles long with 0.2 stop per mile and consists of warmed-up
vehicle operation on a chassis dynamometer through a specified driving
cycle. A proportional part of the diluted exhaust emission is
collected continuously for subsequent analysis of hydrocarbons, carbon
monoxide, carbon dioxide using a constant volume (variable dilution)
sampler. Diesel dilute exhaust is continuously analyzed for
hydrocarbons using a heated sample line and analyzer. Methanol and
formaldehyde samples are collected and individually analyzed for
methanol-fueled vehicles (measurement of methanol and formaldehyde may
be omitted for 1993 through 1994 model year methanol-fueled vehicles
provided a HFID calibrated on methanol is used for measuring HC plus
methanol).

(3) Except in cases of component malfunction or failure, all emission
control systems installed on or incorporated in a new motor vehicle
must be functioning during all procedures in this subpart. The
Administrator may authorize maintenance to correct component
malfunction or failure.

(4) Transmission . The provisions of §86.128 of this chapter apply for
vehicle transmission operation during highway fuel economy testing
under this subpart.

(5) Road load power and test weight determination . §86.129 of this
chapter applies for determination of road load power and test weight
for highway fuel economy testing. The test weight for the testing of a
certification vehicle will be that test weight specified by the
Administrator under the provisions of part 86 of this chapter. The
test weight for a fuel economy data vehicle will be that test weight
specified by the Administrator from the test weights covered by that
vehicle configuration. The Administrator will base his selection of a
test weight on the relative projected sales volumes of the various
test weights within the vehicle configuration.

(6) Vehicle preconditioning . The HFET is designed to be performed
immediately following the Federal Emission Test Procedure, §§86.127
through 86.138 of this chapter. When conditions allow, the tests
should be scheduled in this sequence. In the event the tests cannot be
scheduled within three hours of the Federal Emission Test Procedure
(including one hour hot soak evaporative loss test, if applicable) the
vehicle should be preconditioned as in paragraph (b)(6) (i) or (ii) of
this section, as applicable.

(i) If the vehicle has experienced more than three hours of soak (68
°F–86 °F) since the completion of the Federal Emission Test Procedure,
or has experienced periods of storage outdoors, or in environments
where soak temperature is not controlled to 68 °F–86 °F, the vehicle
must be preconditioned by operation on a dynamometer through one cycle
of the EPA Urban Dynamometer Driving Schedule, §86.115 of this
chapter.

(ii) In unusual circumstances where additional preconditioning is
desired by the manufacturer, the provisions of §86.132(a)(3) of this
chapter apply.

(7) Highway fuel economy dynamometer procedure . (i) The dynamometer
procedure consists of two cycles of the Highway Fuel Economy Driving
Schedule (§600.109(b)) separated by 15 seconds of idle. The first
cycle of the Highway Fuel Economy Driving Schedule is driven to
precondition the test vehicle and the second is driven for the fuel
economy measurement.

(ii) The provisions of §86.135 (b), (c), (e), (f), (g), (h) and (i)
Dynamometer procedure of this chapter, apply for highway fuel economy
testing.

(iii) Only one exhaust sample and one background sample are collected
and analyzed for hydrocarbons (except diesel hydrocarbons which are
analyzed continuously), carbon monoxide, and carbon dioxide. Methanol
and formaldehyde samples (exhaust and dilution air) are collected and
analyzed for methanol-fueled vehicles (measurement of methanol and
formaldehyde may be omitted for 1993 through 1994 model year
methanol-fueled vehicles provided a HFID calibrated on methanol is
used for measuring HC plus methanol).

(iv) The fuel economy measurement cycle of the test includes two
seconds of idle indexed at the beginning of the second cycle and two
seconds of idle indexed at the end of the second cycle.

(8) Engine starting and restarting. (i) If the engine is not running
at the initiation of the highway fuel economy test (preconditioning
cycle), the start-up procedure must be according to the manufacturer's
recommended procedures.

(ii) False starts and stalls during the preconditioning cycle must be
treated as in §86.136(d) and (e). If the vehicle stalls during the
measurement cycle of the highway fuel economy test, the test is
voided, corrective action may be taken according to §86.1834–01 as
applicable, and the vehicle may be rescheduled for test. The person
taking the corrective action shall report the action so that the test
records for the vehicle contain a record of the action.

(9) Dynamometer test run . The following steps must be taken for each
test:

(i) Place the drive wheels of the vehicle on the dynamometer. The
vehicle may be driven onto the dynamometer.

(ii) Open the vehicle engine compartment cover and position the
cooling fan(s) required. Manufacturers may request the use of
additional cooling fans for additional engine compartment or
under-vehicle cooling and for controlling high tire or brake
temperatures during dynamometer operation.

(iii) Preparation of the CVS must be performed before the measurement
highway driving cycle.

(iv) Equipment preparation. The provisions of §86.137(b)(3) through
(6) of this chapter apply for highway fuel economy test except that
only one exhaust sample collection bag and one dilution air sample
collection bag need be connected to the sample collection systems.

(v) Operate the vehicle over one Highway Fuel Economy Driving Schedule
cycle according to the dynamometer driving schedule specified in
§600.109(b).

(vi) When the vehicle reaches zero speed at the end of the
preconditioning cycle, the driver has 17 seconds to prepare for the
emission measurement cycle of the test.

(vii) Operate the vehicle over one Highway Fuel Economy Driving
Schedule cycle according to the dynamometer driving schedule specified
in §600.109(b) while sampling the exhaust gas.

(viii) Sampling must begin two seconds before beginning the first
acceleration of the fuel economy measurement cycle and must end two
seconds after the end of the deceleration to zero. At the end of the
deceleration to zero speed, the roll or shaft revolutions must be
recorded.

(10) For alcohol-based dual fuel automobiles, the procedures of
§600.111(a) and (b) shall be performed for each of the fuels on which
the vehicle is designed to operate.

(c) US06 Testing procedures . The test procedures to be followed for
conducting the US06 test are those prescribed in §86.159 of this
chapter, as applicable.

(d) SC03 testing procedures . The test procedures to be followed for
conducting the SC03 test are prescribed in §§86.160 through 161 of
this chapter, as applicable.

(e) Cold temperature FTP procedures . The test procedures to be
followed for conducting the cold temperature FTP test are generally
prescribed in subpart C of part 86 of this chapter, as applicable. For
the purpose of fuel economy labeling, diesel vehicles are subject to
cold temperature FTP testing, but are not required to measure
particulate matter, as described in §86.210–08 of this chapter.

[71 FR 77933, Dec. 27, 2006]