That seems like too much slush to me (however, I said, I'm no expert). At that amount of difference, I would imagine that would generate a lot of (extra / excess) heat in the converter / transmission. But this transmission is a very typical, run of the mill, solid 727.
And another thing that now makes me think the input and output are close to 1:1 is that when I push the accelerator from cruising along at 3,000 rpm, the acceleration / power / rpm all respond linearly. The rpm doesn't "jump up" above the road speed due to slippage. They stay very linear. With cars like my Camry, if I'm cruising, and then I push the pedal more to accelerate, the rpm increases (initially) more than the speed of the car. It can be heard and witnessed via the gauges. The road speed of the Camry sort of lags and then catches up to the engine rpm. That indicates slippage. This is normal for the Camry.
But the Scamp doesn't do this, at least not in drive, and not when driving along at "drive" speeds. It reacts more linear … as if it were locked (but I know it's not). I've driven lock-up converter cars, and it is very distinct when those converters lock.
So, I'm more inclined to believe this is more tachometer error than slippage.
7milesout