The marathon’s unusual distance of 42.195 kilometers (26.2 miles) originated from the 1908 London Olympics. The race was originally intended to be 25 miles, roughly based on the legendary run from Marathon to Athens in ancient Greece. However, for the London Olympics, the course was extended to allow the race to start at Windsor Castle and finish in front of the royal box at the Olympic Stadium. The precise measurement came out to 26 miles and 385 yards (which is 42.195 kilometers). This specific distance was later officially adopted as the standard marathon length by the International Amateur Athletic Federation (now World Athletics) in 1921. The unusual precision of the conversion (26.2 miles or 42.195 kilometers) is regularly remembered by runners worldwide, and often proudly displayed on bumper stickers and t-shirts by those who have completed the distance. Modern marathons are precisely measured using a bicycle fitted with a special counter and following the shortest possible route a runner could take.

The global transition from miles to kilometers occurred at different times across various countries, largely as part of broader metrication efforts. France adopted the metric system first in the 1790s during the French Revolution. Most continental European countries followed throughout the 19th century. In the 20th century, major transitions include: Australia (1970s), Canada (1970s), New Zealand (1969), and the United Kingdom (partially, starting in the 1970s but maintaining miles for road distances). The motivations for switching included: international standardization, simplification of calculations through a decimal-based system, scientific consistency, facilitation of international trade, and political factors (metric adoption sometimes symbolized modernization or political alignment). Many countries experienced transitional periods with both systems being used simultaneously, and some faced public resistance to the change. The United States remains the most significant holdout, having passed the Metric Conversion Act in 1975 but never fully implementing a mandatory transition from the imperial system, largely due to the high estimated costs of conversion and public resistance to changing familiar measurements.

The difference between US gallons and UK (Imperial) gallons significantly impacts MPG calculations, creating confusion in international fuel economy comparisons. A US gallon equals approximately 3.785 liters, while a UK gallon is larger at 4.546 liters—about 20% more. This means that the same vehicle will show a higher MPG rating when measured in Imperial gallons compared to US gallons. For example, a car rated at 30 MPG using US gallons would be rated at approximately 36 MPG using Imperial gallons, without any actual difference in fuel efficiency. This distinction is particularly important when comparing vehicle specifications between American and British sources, or when using online fuel economy calculators. The confusion is compounded by the fact that both measurements are abbreviated simply as “MPG” without always specifying which gallon is being used. When converting between L/100km and MPG, different conversion factors must be used: 235.214 for US MPG, but 282.481 for Imperial MPG. Most international automotive publications now specify “MPG (US)” or “MPG (UK)” to avoid confusion, and many global markets have transitioned to the more universal L/100km measurement to eliminate this ambiguity entirely.

The mile’s seemingly arbitrary length of 5,280 feet evolved through historical adjustments to harmonize different measurement systems. The original Roman mile (mille passuum or “thousand paces”) was approximately 5,000 Roman feet. When this concept was adopted in England, it initially measured about 5,000 English feet, but variations existed throughout the medieval period. The crucial change came during the reign of Queen Elizabeth I with the Statute of the Mile (1593), which officially defined the mile as 8 furlongs. A furlong (derived from “furrow long”—the length of a medieval plowed field) was standardized at 40 rods, with each rod being 16.5 feet, making the furlong 660 feet. Multiplying 8 furlongs by 660 feet yields 5,280 feet. This definition connected the mile to existing land measurements used in agriculture and property boundaries, making it practical for everyday use despite the non-decimal value. The seemingly odd number made perfect sense in the context of other contemporary English measurements, as it allowed easy conversion between miles and smaller units using whole numbers (1 mile = 8 furlongs = 80 chains = 320 rods). This illustrates how many pre-metric measurements evolved organically from practical applications rather than following a decimalized system.

Car speedometers typically have built-in inaccuracies regardless of whether they display kilometers or miles, but with some key differences between measurement systems. Most vehicle manufacturers calibrate speedometers to read slightly higher than actual speed—typically 2-3% faster—to ensure drivers don’t inadvertently exceed speed limits due to instrument error. This “optimistic” reading is legal under regulations in most countries, which allow speedometers to overstate speed but tightly restrict how much they can understate it. When vehicles are imported between countries using different measurement systems, additional variables can affect accuracy. Vehicles converted from miles to kilometers (or vice versa) using electronic recalibration generally maintain their original accuracy characteristics. However, vehicles with mechanical speedometers that use overlay dials or dual scales might introduce minor additional reading errors due to parallax effects or scale compression. Digital speedometers in modern vehicles can usually switch between units with perfect proportional accuracy. GPS-based speed measurements, increasingly available in navigation systems, typically provide more accurate readings than traditional speedometers regardless of the units displayed, as they’re not affected by tire wear, pressure, or modifications that can influence conventional speedometer accuracy. For legal and safety purposes, drivers should be aware that their speedometer likely shows a slightly higher speed than they’re actually traveling, regardless of which measurement system is used.