40 CFR § 1037.805  Symbols, abbreviations, and acronyms.
The procedures in this part generally follow either the International System of Units (SI) or the United States customary units, as detailed in NIST Special Publication 811 (incorporated by reference in § 1037.810). See 40 CFR 1065.20 for specific provisions related to these conventions. This section summarizes the way we use symbols, units of measure, and other abbreviations.
(a) Symbols for chemical species. This part uses the following symbols for chemical species and exhaust constituents:
Symbol  Species 

C  carbon. 
CH4  methane. 
CO  carbon monoxide. 
CO2  carbon dioxide. 
H2O  water. 
HC  hydrocarbon. 
NMHC  nonmethane hydrocarbon. 
NMHCE  nonmethane hydrocarbon equivalent. 
NO  nitric oxide. 
NO2  nitrogen dioxide. 
NOX  oxides of nitrogen. 
N2O  nitrous oxide. 
PM  particulate matter. 
THC  total hydrocarbon. 
THCE  total hydrocarbon equivalent. 
(b) Symbols for quantities. This part uses the following symbols and units of measure for various quantities:
Table 2 to § 1037.805  Symbols for Quantities
Symbol  Quantity  Unit  Unit symbol  Unit in terms of SI
base units 


vehicle frictional load  pound force or newton  lbf or N  kg·m·s−


axle position regression coefficient.  
α  atomic hydrogentocarbon ratio  mole per mole  mol/mol  1. 
α  axle position regression coefficient.  
α 
intercept of air speed correction.  
α 
slope of air speed correction.  

acceleration of Earth's gravity  meters per second squared  m/s2  m·s−


intercept of least squares regression.  

slope of least squares regression.  

vehicle load from drag and rolling resistance  pound force per mile per hour or newton second per meter  lbf/(mi/hr) or N·s/m  kg·s−


axle position regression coefficient.  
β  atomic oxygentocarbon ratio  mole per mole  mol/mol  1. 
β  axle position regression coefficient.  
β 
intercept of air direction correction.  
β 
slope of air direction correction.  

vehiclespecific aerodynamic effects  pound force per mile per hour squared or newtonsecond squared per meter squared  lbf/mph 
kg·m−


axle position regression coefficient.  

axle test regression coefficients.  

constant.  
Δ 
differential drag area  meter squared  m 
m 

drag area  meter squared  m 
m 

drag coefficient.  

correction factor.  

coefficient of rolling resistance  kilogram per metric ton  kg/tonne  10−


distance  miles or meters  mi or m  m. 

massweighted emission result  grams/tonmile  g/tonmi  g/kgkm. 

efficiency.  

adjustment factor.  

force  pound force or newton  lbf or N  kg·m·s−


angular speed (shaft)  revolutions per minute  r/min  π·30·s−


road grade  percent  %  10−


gravitational acceleration  meters per second squared  m/s 
m·s−


elevation or height  meters  m  m. 

indexing variable.  

drive axle ratio  1.  

transmission gear ratio.  

highest available transmission gear.  

load over axle  pound force or newton  lbf or N  kg·m·s−


mass  pound mass or kilogram  lbm or kg  kg. 

molar mass  gram per mole  g/mol  10−3·kg·mol−


vehicle mass  kilogram  kg  kg. 

vehicle effective mass  kilogram  kg  kg. 

inertial mass of rotating components  kilogram  kg  kg. 

total number in series.  

number of tires.  

amount of substance rate  mole per second  mol/s  mol·s−


power  kilowatt  kW  10 

pressure  pascal  Pa  kg·m−

ρ  mass density  kilogram per cubic meter  kg/m 
kg·m−


payload  tons  ton  kg. 
φ  direction  degrees  °  °. 

direction  degrees  °  °. 

tire radius  meter  m  m. 

coefficient of determination.  

Reynolds number.  

standard error of the estimate.  
σ  standard deviation.  

tire revolutions per mile  revolutions per mile  r/mi.  

tire rolling resistance level  kilogram per metric ton  kg/tonne  10−


absolute temperature  kelvin  K  K. 
T  Celsius temperature  degree Celsius  °C  K273.15. 
T  torque (moment of force)  newton meter  N·m  m 

time  hour or second  hr or s  s. 
Δ 
time interval, period, 1/frequency  second  s  s. 

utility factor.  

speed  miles per hour or meters per second  mi/hr or m/s  m·s−


weighting factor.  

wind speed  miles per hour  mi/hr  m·s−


work  kilowatthour  kW·hr  3.6·m 

carbon mass fraction  gram/gram  g/g  1. 

weight reduction  pound mass  lbm  kg. 

amount of substance mole fraction  mole per mole  mol/mol  1. 
(c) Superscripts. This part uses the following superscripts for modifying quantity symbols:
Table 3 to § 1037.805  Superscripts
Superscript  Meaning 

overbar (such as 
arithmetic mean. 
Double overbar (such as 
arithmetic mean of arithmetic mean. 
overdot (such as 
quantity per unit time. 
(d) Subscripts. This part uses the following subscripts for modifying quantity symbols:
Table 4 to § 1037.805  Subscripts
Subscript  Meaning 

±6  ±6° yaw angle sweep. 
A  A speed. 
air  air. 
aero  aerodynamic. 
alt  alternative. 
act  actual or measured condition. 
air  air. 
axle  axle. 
B  B speed. 
brake  brake. 
C  C speed. 
Ccombdry  carbon from fuel per mole of dry exhaust. 
CD  chargedepleting. 
circuit  circuit. 
CO2DEF  CO 
CO2PTO  CO 
coastdown  coastdown. 
comp  composite. 
CS  chargesustaining. 
cycle  test cycle. 
drive  drive axle 
driveidle  idle with the transmission in drive. 
driver  driver. 
dyno  dynamometer. 
effective  effective. 
end  end. 
eng  engine. 
event  event. 
fuel  fuel. 
full  full. 
grade  grade. 
H2Oexhaustdry  H 
hi  high. 
i  an individual of a series. 
idle  idle. 
in  inlet. 
inc  increment. 
lo  low. 
loss  loss. 
max  maximum. 
meas  measured quantity. 
med  median. 
min  minimum. 
moving  moving. 
out  outlet. 
P  power. 
pair  pair of speed segments. 
parkedidle  idle with the transmission in park. 
partial  partial. 
ploss  power loss. 
plugin  plugin hybrid electric vehicle. 
powertrain  powertrain. 
PTO  power takeoff. 
rated  rated speed. 
record  record. 
ref  reference quantity. 
RL  road load. 
rotating  rotating. 
seg  segment. 
speed  speed. 
spin  axle spin loss. 
start  start. 
steer  steer axle. 
t  tire. 
test  test. 
th  theoretical. 
total  total. 
trac  traction. 
trac10  traction force at 10 mi/hr. 
trailer  trailer axle. 
transient  transient. 
TRR  tire rolling resistance. 
urea  urea. 
veh  vehicle. 
w  wind. 
wa  wind average. 
yaw  yaw angle. 
ys  yaw sweep. 
zero  zero quantity. 
(e) Other acronyms and abbreviations. This part uses the following additional abbreviations and acronyms:
Table 5 to § 1037.805  Other Acronyms and Abbreviations
Acronym  Meaning 

ABT  averaging, banking, and trading. 
AECD  auxiliary emission control device. 
AES  automatic engine shutdown. 
APU  auxiliary power unit. 
CD  chargedepleting. 
CFD  computational fluid dynamics. 
CFR  Code of Federal Regulations. 
CITT  curb idle transmission torque. 
CS  chargesustaining. 
DOT  Department of Transportation. 
ECM  electronic control module. 
EPA  Environmental Protection Agency. 
FE  fuel economy. 
FEL  Family Emission Limit. 
GAWR  gross axle weight rating. 
GCWR  gross combination weight rating. 
GEM  greenhouse gas emission model. 
GVWR  gross vehicle weight rating. 
Heavy HDV  Heavy heavyduty vehicle (see § 1037.140). 
HVAC  heating, ventilating, and air conditioning. 
ISO  International Organization for Standardization. 
Light HDV  Light heavyduty vehicle (see § 1037.140). 
Medium HDV  Medium heavyduty vehicle (see § 1037.140). 
NARA  National Archives and Records Administration. 
NHTSA  National Highway Transportation Safety Administration. 
PHEV  plugin hybrid electric vehicle. 
PTO  power takeoff. 
RESS  rechargeable energy storage system. 
SAE  Society of Automotive Engineers. 
SEE  standard error of the estimate. 
SKU  stockkeeping unit. 
TRPM  tire revolutions per mile. 
TRRL  tire rolling resistance level. 
U.S.C.  United States Code. 
VSL  vehicle speed limiter. 
(f) Constants. This part uses the following constants:
Table 6 to § 1037.805  Constants
Symbol  Quantity  Value 


gravitational constant  9.80665 m·s−


specific gas constant  287.058 J/(kg·K). 
(g) Prefixes. This part uses the following prefixes to define a quantity:
Symbol  Quantity  Value 

µ  micro  10−6 
m  milli  10−3 
c  centi  10−2 
k  kilo  10 ^{3} 
M  mega  10 ^{6} 