My 300SDL has a 2.87 - 2.88 rear-axle ratio I think. Anyway it turns these speeds(cruising with light throttle):

@60 mph 2600 rpm

@70 mph 3000 rpm

@80 mph 3400 rpm

@90 mph 3700 rpm

@100 mph 4000 rpm

@110 mph 4200 rpm

@120 mph 4400 rpm

Haven't been beyond there yet! I ran out of adrenaline. My 350SD tachs exactly the same. Notice how the torque convertor tightens up. The speeds are accurate too.

Here's my M103 300SE with 3.46 rear-axle

@60 3000 rpm

@80 3900 rpm

@90 4800 rpm

Haven't been beyond that speed, yet. Boy, that is one hard working six!

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)