Okay, I will agree with that because I just had an oppertunity to drive another car in which what you describes happens. But in my '97 Volvo 850, this is not the case (and I imagine there are other automatic transmissions out there which behave the same way). The tach isn't fucked up, because you can listen to the engine and when you pop it in N at highway speeds, the engine doesn't slow any faster then when you simply take your foot off the pedal in D. I don't know the technical reasons for this happening, but it does happen.
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Andrew Franklin