Funny you guys mention this.. since I'm having a similar problem.

My car is a Euro 300D (1984), and was converted to a US Speedo for import back in the 80's, I'm guessing. Anyway.. I recently got a GPS and used it to check my speed against my cluster.. what do you know, it's off by about 5-8mph. But wait, it gets weirder..

At 0 - 40mph, it's just about perfect. Speedo will read 35mph, and the GPS confirms I'm going 35. From there, it seems to increase in inaccuracy exponentially. At 55mph, I'm going about 58. At 65, I'm going about 70. At 75, I'm going 81.. and at 80 (if I'm able to get this beast UP to that speed), I'm going nearly 88 mph. What the heck is wrong with this thing? This is a real mind boggler.. and the only thing that *I* ever replaced in terms of the speedometer, is the speedo cable when the original one snapped.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]