My '81 TD is most fuel efficient at about 50-55 mph--that's a little unrealistic for highway driving. Nevertheless, speed has a significant impact on mph.
RPM's definitely impact mileage, diesel or gasser--you can sit in your driveway revving to 3500rpm for 10 minutes and burn a lot more fuel than at normal idle.

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1999 E320 4matic really nice Wagon 135k
1981 300TD loud, rattling family car 180k (TGI sold!)
1985 500SEL gas guzzler 220k (sold)
1978 300D my first Turtlediesel (sold)437k