I'm getting 25 MPG mostly highway on my 98 E320 with 173k miles. If I am burning gas outside of the Chicago area I can get up to 4 MPG better gas mileage.

Based on the age of the plugs I would replace them or at least pull them and inspect for gap and color. Also check the air filter.

Have the wheel bearings been repacked?

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles