For those who travel roads with mile markers, like most of the US Interstate system.
Set your trip odometer at the beginning of your measurement, travel X miles (I like 100 for simplicity). When you get to the 100 mile marker, check the trip odometer.
Say you go 100 and your trip says 104.
Divide your indicate mileage by 1.04 to get the actual distance travelled.
Enter that amount in your fuel log as the TRUE distance travelled.
You must NOT do any Engine OFF coasting or touch the ignition key since any operation of the key COULD stop your odometer from working temporarily. Engine ON coasting is fine.
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