It might be a good idea to replace your O2 sensor anyways, as 60,000 miles seems to be its useful life.

But in my experience and those of others I've talked to, a bad O2 sensor will result in huge drops in mpg, like down to 10 mpg. A drop from 21.5 to 16.8 to me doesn't seem like a big enough drop, but who knows?

It may be the gasoline, it may be harder driving...

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".