I've gone 110 mph in my 300SD absolutely no problems whatsoever. The revs were fine, and it was even up a slight incline. I've got 16" wheels on it, but still.

As for the tach, if you still have the stock and haven't already - try undoing the cap for the sensor. It's on the front of the passenger side wheel well under the hood. Unscrew the cap, and then pull out the pins until the plastic assembly separates. Put some cardboard or something on the back side of the glue/silicone and then put it back in the cap. The cardboard acts as a buffer to push the pins down into the contacts. For whatever reason over time they don't seem to make contact and this is a really common fix. Worked intermittently on my old junker '83, and seems to be working absolutely perfect on my '81.

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1981 300SD - 283,000 KM's at purchase, 360k+ now. Engine replaced at 311k. 16" CLK wheels, w126 gen II cosmetic upgrades, late w126 leather interior. RIP. Parted and gone due to fire

1987 300SDL - 243K miles and counting. In winter hibernation!

2001 Ford F250 Super Duty "Platinum Edition" Lariat 4x4 7.3L turbo diesel, 295k+ miles, various mods for reliability and performance.