I think the cylinder rating may just refer to its factory adjustment setting. The adjustment is made with a small potentiometer on the back side of the instrument face. (Some are more accessible than others.)

I base this on having adjusted a tach that (I think) was marked as a 6 cyl. to work in my V8 conversion. I did a jumper wire hook-up under the hood, and with engine running, adjusted it to match a digital meter tach.

I also think I once adjusted one "downwards" to work in a '78 240, but that was too long ago to swear to.
--
Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Sold 5/03)
'83 Turbo 245
'76 244 (lasted only 255,000 miles)
73 142 (98K)
'71 144 (track modified--crusher bound)
New 144 from '67 to '78
Used '62 122 from '63 to '67