881 / 2440
12-20 Edition 47
On-Highway Trucks
Engine Selection
Air Resistance:
The HP required to overcome wind resis-
tance. This starts becoming significant
above 30 MPH. Over 65 MPH it can be
the largest single load.
Pa =
FA × Cd × MPH
3
156,000
Cd = Aerodynamic Drag Coefficient. See table below.
FA = Frontal Area of the Truck (sq ft) + Area of the
body or trailer that is wider or taller than the
Truck. If the trailer is more than eight feet behind
the Truck, FA = the Frontal Area of the Truck +
the Total Frontal area of the Trailer. See illustra-
tion below.
156,000 = Constant.
MPH = Desired Cruise Speed (not maximum speed).
Gradeability:
The reserve HP required to operate indefi-
nitely (without downshifting) at the desired
cruise speed on a small grade between
0.5% – 1% Grade.
Pg =
G × GW × MPH
37,500
G = Grade Percent (Typically 0.5 or 1).
GW = Desired Gross Vehicle Weight or Gross Combi-
nation Weight.
37,500 = Constant.
MPH = Desired Cruise Speed (not maximum speed).
Aerodynamic Drag Coefficients
Configuration
Factor
HD Tractor — Full Aero/Van Trailer Full Aero
0.42
HD Tractor — Full Aero/Van Trailer Typical
0.48
MD Van Truck — Full Aero
0.50
HD Tractor — Full Aero/Van Trailer some Aero
0.54
HD Tractor — Full Aero/Tank Trailer Insulated
0.55
HD Tractor — Full Aero/Flat Trailer some Aero (Smooth Load)
0.55
HD Tractor — Full Aero/Van Trailer no Aero
0.80
HD Tractor — Full Aero/Flat Trailer some Aero (Rough Load)
0.80
HD Tractor — Full Aero/Tank Trailer Non Insulated
0.80
HD Tractor — No Aero/Van Trailer no Aero
0.80
MD Van Truck — No Aero
0.80
HD Dump
0.90
MD Dump — No Aero
0.90
HD Tractor — Car Hauler
1.00
HD Tractor — No Aero/Flat Trailer some Aero
1.00
Aero refers to side panels and deflectors to make the Truck more aerodynamic.
73 sq ft
CT660
8.4 ft
8.7 ft