Wayward Tach

Based on advice from a column I read on AVWEB by John Deakin, I had my
Tach checked since it was in the shop anyay. I was surprised to find
out that it was reading 150 RPM lower than the true RPM. That seems
pretty bad. I said, OK, no problem, just adjust it. No dice. I was
informed that it was a "mechanical type" tachometer and that there was
no adjustment possible. I really do not care for the idea of having to
remember to add 150 to the tach reading everytime I do my settings. Is
there really no way to adjust these things? If not, is there an
alternative?

I read an add for a digital tach called a P-1000 by Horizon Instruments.
Seems really good, but at $510, it seemed a bit steep to avoid doing a
little math everytime I set my RPMs. Are there alternative replacement
tachs that are less expensive that folks here have been happy with?

-Sami
N2057M, Piper Turbo Arrow III