Actually, a speed head calibrated for a 3.27 final drive will read high when driven by a 3.46 final.

You are correct!

Say tire revs/mile are 850, then the revs at 60 MPH in a direct drive gear = 850(3.46)=2941

With a 3.27 axle revs at 60 = 850 (3.27) = 2780, and if the speedo gearing is set up for a 3.27 axle the speedo will read 60 at 2780 regardless of the installed axle ratio.

So with an installed 3.46 axle and the speedo gearing set up to read 2780 at 60, actual speed at 2760 is 2780/2941(60) = 56.7, so the speedometer is reading 60/56.7 = 1.058, which is about six percent high, and this percent error is constant over the entire speed range.

Duke