Vehicular metrics
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There are a broad range of metrics that denote the relative capabilities of various vehicles. Most of them apply to all vehicles while others are type-specific.
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, cargo capacity | 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 |
Rate of climb | feet/min | ? | combat effectiveness, economics | higher is better | Applies to fighter aircraft who need to intercept or evade other fighters. In civilian aircraft this denotes how quickly they can reach optimal cruising altitude. |
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 |