This is unusal. I used to get 17-18 mpg under city-highway driving on my 300, but lately I am getting over 20 mpg in the last 4 months, what gives?

Can a car get better gas mileage with age? Also, I need to change my spark plugs because they are older than a year with > 20,000 miles on them, but the car is running better than after last tuneup. What gives?

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)