Done that. The tach goes to an external interrupt pin. I put a 10k resistor in series, a 5v zener from the pin-to-ground to avoid engaging the protection diodes, and a 50k in parallel with the zener to avoid triggering on noise if the wire is unhooked.
I used a graphics LCD with a large graphical font and a bar display myself.
Put the TMR interrupt and tach interrupts at a higher priority than whatever writes the LCD. That could mean high vs low priority interrupts, or using unprioritize interrupts for those and put the LCD writing in the main body. The rpm value will need protection so it doesn't get rewritten while it's being sent to an LCD, generally you'd disable the timer interrupt, quickly copy the value, and reenable it. If prioritized interupts are used, the TMR interrupt will need to disable the tach input's interrupt too.