things aren't adding up...

[GMC Guy]

i'm really not sure what is reading inaccurate. i'd just like to know how fast i'm going lol

1. just replaced the speedo cable, re-greased, no more boucing
2. 28 inch tall tires, bfg 275/60/15
3. doubled checked rear gear ratio. keep coming up with 3.08

here's where i'm confused, when i use the RPM calculator, it doesn't match up with what my tach and speedo are showing me on the road. (new Auto Gage tach btw)
http://www.csgnetwork.com/multirpmcalc.html

according to my gauges, here's what i'm doing on the freeway:

65mph: about 2700 rpm
70mph: about 2850 rpm
75mph: about 3000 rpm
80mph: about 3300 rpm

these numbers don't add up with what i should be doing with 3.08's, but they are almost the same as the numbers for 3.42 gears...

sorry if i'm not making sense guys, just confused
just looking for anyone's advice on what isn't reading accurate, thanks ahead of time