Automotive speed measurement balances legal requirements, safety, and performance testing across different regional standards.

• **Global speed limits:** Urban 30–60 km/h (20–37 mph); highways 80–130 km/h (50–81 mph); Germany's Autobahn has unrestricted sections
• **Performance benchmarks:** 0–100 km/h (0–60 mph) acceleration is the industry standard; supercars achieve this in under 3 seconds
• **Speed enforcement:** Radar guns measure speed using Doppler shift; typical accuracy ±2 km/h (±1 mph)
• **GPS speedometers:** More accurate than mechanical speedometers (which can read 5–10% high for safety margins)
• **Racing circuits:** F1 cars reach 370 km/h (230 mph); top speeds limited by drag, downforce trade-offs
• **Electric vehicles:** Instant torque enables faster 0–100 km/h than comparable ICE vehicles despite often lower top speeds

Aircraft speed measurement distinguishes between indicated airspeed (IAS), true airspeed (TAS), and ground speed (GS) — critical for safety and navigation.

• **IAS (Indicated Airspeed):** What the pilot sees; based on dynamic pressure. Used for aircraft performance limits (stall speed, max speed)
• **TAS (True Airspeed):** Actual speed through air mass; higher than IAS at altitude due to lower air density. TAS = IAS × √(ρ₀/ρ)
• **Ground Speed (GS):** Speed over ground; TAS ± wind. Tailwinds increase GS; headwinds decrease it. Critical for navigation and fuel planning
• **Mach number:** Aircraft performance changes dramatically near Ma = 1 (transonic region); shock waves form, drag increases sharply
• **Airliner cruise:** Typically Ma 0.78–0.85 (optimum fuel efficiency); equals ≈850–900 km/h (530–560 mph) at cruise altitude
• **Military jets:** F-15 max speed Ma 2.5+ (2,655 km/h / 1,650 mph); SR-71 Blackbird held Ma 3.3 (3,540 km/h / 2,200 mph) record
• **Re-entry speeds:** Space Shuttle entered atmosphere at Ma 25 (8,000 m/s, 28,000 km/h, 17,500 mph) — extreme heating requires thermal protection

Maritime speed measurement uses knots and nautical miles — units tied directly to Earth's geometry for seamless chart navigation.

• **Why nautical miles?** 1 nautical mile = 1 minute of latitude = 1,852 meters exactly (by international agreement 1929). Makes chart plotting intuitive
• **Origin of knots:** Sailors used a 'log line' with knots tied at regular intervals. Count knots passing over stern in fixed time = speed in knots
• **Ship speeds:** Container ships cruise at 20–25 kn (37–46 km/h); cruise ships 18–22 kn; fastest passenger ship (SS United States) reached 38.32 kn (71 km/h)
• **Current effects:** Gulf Stream flows at 2–5 kn eastward; ships utilize or avoid currents to save fuel and time
• **Dead reckoning:** Navigate by tracking speed and heading over time. Accuracy depends on precise speed measurement and current compensation
• **Speed through water vs over ground:** GPS gives speed over ground; log measures speed through water. Difference reveals current strength/direction

Scientific measurements use m/s and reference speeds that define physical regimes — from molecular motion to cosmic velocities.

• **Speed of sound (air, 20°C):** 343 m/s (1,235 km/h, 767 mph). Varies with √T; increases ~0.6 m/s per °C. Used to define Mach number
• **Speed of sound (water):** ≈1,480 m/s (5,330 km/h) — 4.3× faster than air. Sonar and submarine detection rely on this
• **Speed of sound (steel):** ≈5,960 m/s (21,460 km/h) — 17× faster than air. Ultrasonic testing uses this for flaw detection
• **Escape velocity (Earth):** 11.2 km/s (40,320 km/h, 25,000 mph) — minimum speed to escape Earth's gravity without propulsion
• **Orbital velocity (LEO):** ≈7.8 km/s (28,000 km/h, 17,500 mph) — ISS orbital speed; balances gravity with centrifugal force
• **Earth's rotation:** Equator moves at 465 m/s (1,674 km/h, 1,040 mph) eastward; used by rockets launching east for velocity boost
• **Speed of light (c):** 299,792,458 m/s exactly (by definition). Universal speed limit; nothing with mass can reach c. Time dilation occurs at relativistic speeds (>0.1c)
• **Particle accelerators:** Large Hadron Collider accelerates protons to 0.9999999c (≈299,792,455 m/s) — energy increases dramatically near c

• **Furlong per fortnight:** Humorous unit = 1 furlong (⅓ mile) per 14 days ≈ 0.000166 m/s (0.6 m/h). Used in physics jokes and Douglas Adams' works
• **League per hour:** Medieval travel speed; 1 league ≈ 3 miles (4.8 km), so 1 league/h ≈ 1.3 m/s (4.8 km/h) — typical walking pace. Appears in Jules Verne novels
• **Roman pace (passus):** Roman mile = 1,000 paces (≈1.48 km). Marching legions covered 20–30 Roman miles/day (30–45 km/day, ≈1.5 m/s average)
• **Verst per hour (Russian):** 1 verst = 1.0668 km; used in 19th century Russia. Train speeds quoted in versts/hour (War and Peace references)
• **Li per day (Chinese):** Traditional Chinese li ≈ 0.5 km; long-distance travel measured in li/day. Silk Road caravans: 30–50 li/day (15–25 km/day)
• **Admiralty knot (pre-1954):** British definition 6,080 ft/h = 1.85318 km/h (vs modern 1.852 km/h). Small difference caused navigation errors; standardized in 1954

• m/s × 3.6 → km/h; m/s × 2.23694 → mph
• Round sensibly for road/aviation reporting
• Use significant figures for scientific work

• 1600s
  Log line with knots used at sea to estimate speed
• 1900s
  Automobile speedometers become common
• 1947
  First supersonic flight (Bell X‑1)
• 1969
  Concorde’s first flight (supersonic airliner)
• 1997
  ThrustSSC breaks the sound barrier on land