If you look at "mileage" as a compound word, it turns into "mile age", which means the "age of the miles you've driven". Basically it means how many miles you've gone. If you remember the keyword "mile", it'll help you associate the word "mileage" to a car. I think that made sense.
The other answers don't give you hints on how to spell the word, it only gives you information on what the word means.
