The tach that came with my stock 305 read 3,000 rpm at 60mph. So, I went to a junk yard and got a tach out of the same year camaro but a V6. That tach reads 2,500 at 60mph. That still seems high with just having 2.73 gears. The 305 has been swapped out for a 383 and tranny is a TH350. At any rate, it seems unlikely that 2 tachs would read wrong. Could it be something else throwing it off? Would a reliable tach for V8 read lower than the V6 tach?

 

I am pretty sure that the V6 tach reads the tach signal differently then the V8 tach. It measures the pluses from the coil differently.

 

The tachs count iginition pulses.

4 / revolution for the V8, 3 / revolution for the V6.

A V8 tach in a V6 will read 3/4 the actual engine RPM.
A V6 tach in a V8 will read 4/3 the actual engine RPM.

So yes, regardless of engine, a V8 tach will read lower RPM than a V6 tach. If it doesn't, one of your tachs is messed up. It sounds like the V6 tach might be the accurate one (or close to it); that would equal 1875 RPM at 60mph.

A quick check gives me 1600 RPM in 4th gear with the converter locked at 60mph; that was with 245/50R16 tires. 3rd gear would be 2100rpm (same speed, same tires). 200 or so RPM off (for the V6 tach) is not outside the realm of possibility.

Your V8 tach is definitely wrong.