As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOTS FELT ENGINE WAS NOT PRODUCING FULL POWER RETURNED TO BASE. ENGINE INSPECTED BY AN APPROVED AVIATION COMPANY, ALL ENGINE PARAMETERS WERE NORMAL. HOWEVER, THEY DISCOVERED THE TACHOMETER WAS 100 RPM LOW AT FULL POWER. REPLACED TACHOMETER.