These cars never got 18-22 mpg, even when new. 15 mpg on the high end and 12 is normal average in mixed driving. 10 mpg in the city is common. None of the more than dozen 225 slants I've owned ever got better than 15mpg, and that was only after looong trips at steady speeds.
I think you are trying to check your mileage too frequently. Average it out over about 500 hundred miles to see what you are really getting. Too much error in a 130 mile trip.
