How do you know when it is time to change your engine oil? Maybe there is a sticker on your door that is visible when you open the door and crane your neck down to waist level. But, you still need to subtract the mileage figure on the sticker from the reading on your odometer. It can work, but it is not a great system.
Step 1: Use the Change Oil Light?
Modern automobiles have a Change Oil light on the instrument display. See the green box. It lights when it is allegedly time to change your oil. But, it is only a computer's guess that it is time to change your oil. Your engine's computer notices the amount of city driving you do in comparison to highway driving and guesses that your oil has reached a certain level of contamination from engine wear, water vapor in the crankcase, acids, and engine blow-by gasses. A truly accurate assessment of your oil's condition would require testing in a laboratory.
The Low Tire Pressure light next to the Change Oil light also does not really know if one tire is low. It makes a secondary measurement that is usually explained by low tire pressure. It compares the number of revolutions each tire makes. If a tire is low, it has a smaller effective diameter and must turn more times per minute to keep up with the fully inflated tire on the opposite side of the car. My Low Tire Pressure light has remained "on" since a mechanic worked on the braking system a couple of years ago.