Its all computed inside the computer, based upon how long the engine is running, speed, idle etc. So results will very, I do 80% highway at 60 MPH so I would expect someone who is in the city to have shorter intervals. I have had a variance of about 3000 miles on them, of course one time I let it get below zero and that is very annoying. It will beep at you every couple miles.
Chris
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2010 I2 "the silver slope"
2005 Element
lots of cast iron in the basement.
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