Need help solving 61 drivetrain / RPM problem

[SMR]

Ok guys I could use some real help on this problem. Go easy on me if it’s something stupid. Here is my problem. I have a 61 C10, 283 stock motor with 2 speed powerglide transmission, running 29 inch tires. I’m running about 2k rpms at 40 mph and wanted to step down the rpms by changing the rear end gear set to a 3.38. Based on conversations with Ring&Pinion.com, I was told I had at least a 3.90 set in the rear end and that going to 3.38 would lower the rpms to a more highway friendly 2600 rpms at 65. The killer is that when my mechanic opened up the rear end the gear set was already 3.38 (44/13). Something doesn’t make sense as I should be running at around 1600 rpms at 40 mph. I had the trucks digital dash, which displays rpm checked for accuracy, by having my mechanic connect up his own equipment to see if there was a problem with the rpm reading. Both were inline with one another. According to my mechanic the transmission is shifting fine and does not appear to be slipping. Any ideas?