alf5something,
Yes you are correct. V1 is calculated and it it is what it is.
There are many failures that are practised in the sim and there are rejects and there are continues.
However, its very much not as cut and dry as that.
This example is a classic if it is what it has currently been reported as.
At 80kts, both speed tapes are spot on in agreement.
At 120kts, there is a sudden and dramatic disgreement.
V1 = 125kts.
Edited to say that in general, an IAS disagree that is spotted, diagnosed and countered using the Sby ASI wouldn't be something i'd stop for at high speed.
(All numbers made up)
You can stop or you can go.
I was exaggerating the point that on a long runway with awful weather id be inclined to stop.
On a short runway with good weather id be inclined to go.
IN THE MOST SIMPLISTIC OF TERMS.
The rest is a judgement call. It gets made and then god willing, you only have to suffer trial by PPRUNE.