Seems like this thread has too many comments from east coast and upper MW drivers, used to low speed limits, rough roads and rusted out hulks, roads clogged with semis, etc.

The stretch of road the OP is talking about probably has an average speed in the upper 80s. There are many roads out west with 80 mph speed limits, where average speeds are near 90, the road surface is as smooth as glass, your in a dry desert with 100 mile visibility, etc.

I suspect much of the instability problem he felt while performing this run had to do with the famous down-slope wind you get on the east side of the Rockies.

My 240D feels perfectly stable at 90, and I get passed by many cars when I'm driving that fast out here in the wide open west.

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1979 240D, 4spd manual, Power Sunroof, manual windows, 147k miles, Pastel gray/Black MB Tex.
1991 300D 2.5 - Smokes like it's on Crack!