That gauge is known as an odometer. The name comes from the late 18th century: from French odomètre, from Greek hodos ‘way’ + meter.
The accuracy of an odometer gauge has changed over the years. Today it’s electronic, whereas years ago, a cable ran from the speedometer head down to the transmission. As the transmission rotated, this cable rotated and drove a mechanical set of wheels that displayed the appropriate numbers. Because of that, odometer readings could be easily altered. That’s not possible with the LED-displayed odometers we have today.
The odometer gauge accurately shows how many miles are on a car but has other uses too such as calculating fuel mileage and distances traveled in a certain time frame. To calculate accurate fuel mileage simply record the mileage reading when you fill up with fuel and then again when you next fill up. Determine the number of miles driven and divide that by the number of gallons used in the second fill up. If you do this several times and use the total miles from start to finish and divide by the total gallons used during that period, you will get the most accurate miles per gallon result.
On a highway, you can carefully watch the mile markers and the odometer reading to figure out if your odometer is reading accurately. Travel four miles watching the odometer reading as you pass the first mile marker and the fourth. The tenths of miles should match precisely. If not, it is possible there is a malfunction or the most likely scenario is that a change in tire size has caused the odometer reading to be somewhat inaccurate.
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