So, I have no idea what a tachometer is used for when driving a car, other than “don’t let it get into the red”. Diagnostics, though, here’s a story.
So I’m driving home, and my car is acting a bit funny. Engine runs smooth, but every time I let off on the gas power plummets almost fast enough to feel like a stall, but there’s no shuddering as the engine struggles before giving out. Like highway to idle in an instant is what it felt like. Didn’t notice anything on acceleration.
Running through everything it could be, ruling out all the easy things I can handle on my own, then I catch a glance of my tach. About 2.7k rpm at about 35mph.
So then I nearly shit a brick. This held, it wasn’t a brief spike during acceleration. It was my cruising RPM at that speed. I was thinking my transmission died and couldn’t reach the gears it was trying for. Bad news.
Then I remembered that even automatics have some ability to override the transmission and set a specific gear, and sure enough, I was set to 2 rather than D. Set it properly, and my car ran fine.
I’m not sure I’d have thought to check anything transmission related if not for the tach, which would lead to me being home wondering what the hell was wrong with my car, and being unable to reproduce the problem when my dad came down to look at it.
2.7 at 35, though, makes me wonder how high my engine would go if I went on the highway in 2nd. I redline at 6.5k(never seen past 4 even in hard acceleration) so I probably wouldn’t immediately break anything, but I can’t imagine sustained driving past the ~2.5k I typically see on the highway would be all that great for engine lifespan.