When my speedo went out in my 190E, I would use 3000 rpm for 60 mph. I would just double the rpm reading. The actual for a reading of 30 would be about 57-58 mph but I would just double the rpm reading for safety. If the speed limit was 50, I would go no more than 25 (2500 rpm).

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)