HOW ACCURATE ARE THE FACTORY TACHS

IMO
for a streeter;
never mind the accuracy,the factory location is terrible.
To see it, you have to take your eyes off the road, and re-focus, and your eyes have to readjust to the different lighting.
Then, you look back to see where you are going, and your eyes have to readjust again.As your eyes get older, it takes longer for them to do this.
Top of second gear might be 90 mph, and you are moving at 132 feet per second. Even with young eyes, this business might take .25 second/33ft, to .5 second/66 feet. A two-lane hiway is 24 ft wide,36 with graveled shoulders up here in Manitoba.How do you feel about that?
IMO
the tach should be on top of the dash, in or nearly in, your line of sight. and with a shift light, so you never have to focus on it. And if you're buying new, I recommend a 3.75 inch 270* sweep, with rpm suited to your redline. THat way you get the best resolution. Those big racing tachs are mostly 11,000 or 8000 rpm jobs; and street-engines are mostly 5000 to 6000, occasionally 6500
But for a streeter, it doesn't have to be particularly accurate, cause you probably don't have a dyno chart anyway, so you are just guessing about when to shift. And even if you knew where the power plateau was, it can be 300/400 rpm wide, which at the top of second gear might be 6 to 8 mph wide........ so, then, you might as well just mark the speed-O meter,lol, and use it.
Honestly; I rarely look at my tach. I just floor it and let her buck. Somewhere between 5 and 6 seconds later, the car is getting close to speeding, so I shift into second and shut down.
My cheap tach has a rev-limiter in it, that I find very useful as a cruise control. I set it to whatever rpm gets me 100kph/62 mph and that eliminates the possibility of me getting a speeding ticket. Well it outta.... and it must be working, cuz that car has never since it first rolled out in 1999,got me a speeding ticket.
It has a tiny adjustable shift light too, which I mostly ignore and drive right on thru; it seems like I got some of my Momma's rebelliousness in me.
Nice tach, but mostly jewelry.