http://www.wallaceracing.com/calc-rgr.php
Calculate Rear Gear Ratio
for given MPH, RPM and Trans Gear Ratio
When I plug in the numbers but in the trans gear ratio I put the th350 2nd gear ratio of 1.52
I get a more normal 3.66 and with the speedo being off, your tach being a tad off..
and me using 28" tire not 28.5"
it get close to a g.m. 3.42 rear gear..
So.. let me ask, are you sure it's going into high gear.. and not going down the highway in 2nd