W210 E420 - sudden MPG drop

[Glen]

My sister-in-law's '97 E420 with M119 has suddenly been getting really low gas mileage on long freeway drives. Historically, it has returned about 20 to 21 MPG in mixed town/highway and 27 to as much as 30 MPG on long trips.

For the last two months or so, highway mileage is down to 20/21 but intown mileage has not really changed...in effect, no matter what type of driving is done, the car is returning about 20 to 21 MPG.

The car drives fine, engine has good power, idles smooth and accelerates normally. No CEL.

What could be wrong?

Thanks!