Alright, I'll take a stab at this one since I have a C6 as well. These formulas are perfect world in nature, so assume real world results will be slightly different.
215/85R16 tires = 30.4" diameter, or 664 revolutions per mile/5,280 feet.
664 tire revolutions x 3.55:1 differential = 2357.2 driveline revolutions per mile.
60 mph is the best place to calculate rpms, because it's when the driveline turns those 2357.2 revolutions in a minutes time, RPM'S...
With a manual trasmission, theres no slip. But a C6 has torque converter 'slip', and is not a true 1:1 final ratio.
At 60 mph, your driveline is turning 2357.2 rpms. Now to get to the engine rpms, time to figure in the torque converter slip. I know from owning two C6's that typical slip at full speed is between 2% & 7%. I owned an '85 that had a 6.5% slip, my '88 has a 4.5% slip ratio. I'm gonna use 5% as middle ground here.
2357.2 divided by 0.95 = 2481.26 engine rpm's at 60 mph. I'll confirm this, since when my truck had 3.55's, my tach showed 2500 rpms at ~60mph (part of the needle touching on both gauges at the numbers mentioned.) Since you're using a gps, speedo-correctness is out of the conversation.
If I plug 4.10 into the formula, I get 2865.68
If I plug 3.73 into the formula, I get 2607.07
Ford only had a 3.55 or a 4.10 option, but either way I'm not getting anything within any margin of error to 2700 rpm. I've seen tach's show slow on rpms, or be double, but never just a 'hair' fast. Not saying it's not possible, just haven't seen it. (If anyone has, speak up). Maybe the GPS was having a moment. I'd find a nice straight open stretch and see if you can hold 60/2700 for an extended period and see if it seems to be what the truck and gps want to tell you.
My truck came with 265/75R16 tires and 4.10's. I plugged that into the this formula and got 2702 RPM'S at 60 mph...