Not sure if this link has been posted already but I couldn't find anything when I searched for it. My tach won't rev up past 3000 or so, I hear this is a common problem on MR2's also. This describes the process of fixing the problem caused by two worn out capacitors. I haven't done it yet but will in the near future. Hope it helps!

http://mr2.noegruts.com/tachometer/tachometer.htm