Vehicular metrics
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 100 km/h (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 |
| autonomy | kilometers | comfort | higher is better | ||
| 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 | |
| traveled Distance | kilometers | economy | higher is better, lower is better for a second hand acr | ||
| 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, braking (safety) | higher is usually better | measures cornering, braking or forward acceleration |
| Ground pressure | psi | pascals (sometimes bar) | 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 |
| Noise | dB | dB | comfort | lower is better | |
| Torque | lbf·ft or lb·ft | N·m | acceleration | higher is better | Refers to the overall maximum torque an engine can produce, or the maximum torque an engine can produce at a given RPM. 300 lbf·ft would be like applying 300 pounds of force to the end of 1 foot long wrench, or twisting a 2-inch diameter shaft with 3600 pounds of force! |
| Power | hp | kW | acceleration | higher is better | Refers to maximum power (high torque and speed). The rate at which torque is applied. Also the rate at which work is done. Power = Torque × RPM / 5252. Automobile manufacturers publish power measured at the crankshaft (bhp or brake horsepower). However, it is the power a car can produce at the wheels (wheel horsepower or whp) that matters when it comes to acceleration performance. Wheel horsepower equals brake horsepower minus drivetrain losses, which can be anywhere from about 10% to 25%[1]. |
| 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 | meters/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 |
| Size | - | meters | handling, safety | lower is better for parking on narrow parking slots, higher is better for lateral Traffic collision | |
| 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)
A speed greater than the legal maximum/recommended speed limit can be considered as useless/unsafe. |
| Turning radius | feet | meters | handling | lower is better | |
| Weight, mass or Dry 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 |
References
- "Drivetrain losses (efficiency)". X-Engineer.org. Retrieved 2019-01-01.
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