I don't know about the 79-80SD but I replaced my 81 300D cluster w/ no problems. The old spedo maxed at 85mph and the new one is 140. The readings were fine w/ no problem on all gauges, including spedo.

clint

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81 300D N/A 130K miles (Nelly)
87 300D Turbo 80K miles (The Scalded Dog...because it runs like a scalded dog)
95 e320 Wagon 104K
2003 Sprinter 47K miles.
1999 Tandem Bicycle
One beautiful low miles wife (who likes diesels) (that's my wife holding the sign)

My son and I fixing Nelly's odometer