Or could it have been an incorrect tach reading?
Sure it could, but since it's A minus B, as opposed to absolute numbers, it goes out in the bath water
Say you had 3.23s and 27" tires. Zero-slip at 30mph in drive would math out to 1204 rpm. Now say you had a 1969 hi-torque 318, or a maybe a 5.2 Magnum. So then, say you are cruising on level ground. Now what rpm might you expect on the tach? Well firstly that rpm is well below stall, and secondly, the teener is well be low it's torque-peak; so the tach could be saying anything, well, not anything exactly,lol.
Let's go to 45mph. Zero-slip maths to 1806. And the TC stall is about 1750. So now the teener is working a bit, and still below it's max efficiency, so I would expect the Rs to be closer to 1806, but perhaps a little higher.
Finally, lets move up to 60 mph. Now zero-slip would be 2408. This is about where the teener makes peak-torque , and she is well past the TC stall. The teener is now loafing, cuz she has very little work to do, just pushing a bit of wind. This is the times I have seen the tach lower than zero-slip. But if I step out to pass, the tach pops up right away, to greater than zero-slip. At 5% slip I might expect the tach to pop up 120 rpm, with throttle application. But if I now back out of the throttle, the Rs might fall 300 to 400 from that 2520, to say 2170, yet still at 60mph. That's a negative slip number of 10% from 2408, the zero-slip number.
The 318, cruising at peak-torque like this, could put the tach anywhere between these numbers depending on the throttle position, yet always at 60mph.
Say she is climbing a steep hill; the tach could be at the max of 2520. Or say she is going downhill at reduced throttle; it could be on the minimum of 2170, or possibly even less.
Say you kept the tach on the zero-slip number, and just let the car accelerate until it stops accelerating.What would the speed be? Who knows,lol. At under 60mph on hard level ground, I doubt you would ever find it. I imagine there would be a speed somewhere were she would find a zero-slip equivalent, but I suspect it would be higher than 60mph. I've never looked for it. I just found it interesting, and worth noting.
