| Measurement
|
American unit
|
Metric unit
|
Affects
|
General preference
|
Notes
|
| 0 to 60 mph
|
seconds
|
seconds
|
acceleration
|
lower is better
|
|
| 0 to 100 to 0 mph
|
seconds
|
seconds
|
acceleration & braking
|
lower is better
|
formerly common in British publications
|
| Braking distance
|
feet
|
meters
|
safety
|
shorter is better
|
|
| Brake specific fuel consumption
|
lb/(hp·h)
|
g/(kW·h)
|
economics, range
|
lower is better
|
|
| Drag coefficient
|
(ratio)
|
(ratio)
|
economics, top speed, range
|
lower is better
|
|
| Frontal cross-section area
|
sq ft
|
m2
|
economics, top speed, range
|
lower is better
|
if area is too small, vehicle becomes difficult to use
|
| Fuel economy
|
mpg
|
l/100 km and km/L
|
economics, range
|
greater is better (mpg and km/L), lower is better (L/100 km)
|
must be specified on new vehicles for sale in the US & UK
|
| Maximum g-force(s)
|
g or ft/s2
|
g or m/s2
|
acceleration
|
higher is usually better
|
measures cornering, braking or forward acceleration
|
| Ground pressure
|
psi
|
pascals
|
traction
|
lower is better in soft ground, reduces bogging; higher with loose surface
|
has greater impact on off-road vehicles
|
| Lift to drag ratio
|
-
|
-
|
economics, range
|
higher is better for aircraft
|
improved by narrow, long wings
|
| Power
|
hp
|
kW
|
acceleration
|
higher is better
|
Refers to maximum power (high torque and speed)
|
| Power-to-weight ratio
|
hp/lb
|
W/kg
|
acceleration
|
higher is better
|
|
| Second moment
|
psi (lb·sq ft)
|
kg·m2
|
handling
|
lower permits quicker turn-in for cars, higher is more stable in straight line.
|
The moment of inertia about a vertical axis of a vehicle
|
| Propulsive efficiency
|
%
|
%
|
economics, range
|
higher is better
|
For rockets and aircraft, percent of the energy contained in a vehicle's propellant converted into useful energy,
|
| Roll center
|
inches
|
mm
|
handling
|
Too many variables to state a general preference.
|
|
| Rolling friction
|
-
|
-
|
economics
|
lower is better
|
improved by narrow, high pressure tires
|
| Shift time
|
mSec
|
ms
|
acceleration
|
lower is better
|
for vehicles equipped with automatic transmissions
|
| Specific fuel consumption (thrust)
|
lb/(lbf·h)
|
kg/(kgf·h) or g/(kN·s)
|
economics, range
|
lower is better (for any given speed)
|
in airbreathing jet engines it is improved by using more inert air for propulsion (i.e. lower exhaust velocity), in rockets, higher exhaust velocity
|
| Specific fuel consumption (shaft engine)
|
lb/(hp·h)
|
kg/(kW·h)
|
economics, range
|
lower is better
|
for shaft engines less fuel use for a given output power means higher efficiency
|
| specific impulse
|
seconds
|
seconds or kN·s/kg
|
economics, delta-v/range
|
higher is typically better
|
in airbreathing jet engines it is improved by using more inert air for propulsion (i.e. lower exhaust velocity), in rockets, higher exhaust velocity
|
| Top speed
|
mph
|
km/h
|
Maximum rate of straight line travel
|
higher is better
|
Electronically limited in some cars for safety (mostly due to concerns of tire failure at high speed)
|
| Turning radius
|
feet
|
meters
|
handling
|
lower is better
|
|
| Weight
|
lb
|
kg
|
acceleration, braking distance, traction, fuel consumption, tyre wear
|
lower is better for vehicle performance and taxation; larger is usually better for vehicles carrying loads
|
|
| Weight distribution
|
%
|
%
|
handling, acceleration, traction
|
close to 50:50 (%Front:%Rear) is commonly considered better
|
|
| Gross axle weight rating
|
lb/axle
|
kg/axle
|
durability, economics
|
larger is better for vehicles carrying loads
|
Ultimately limited by the hardness of the road surface and legal limits intended to limit damage to it
|
