If I get to 120mph, I'm not looking at my tach...
But here's a calculator to figure it out if you want to: http://www.bgsoflex.com/rpmmph.html
Looks like that calculator is designed for automotive type vehicles, not Harley's. No place to input the primary drive ratio.
The final drive (belt) ratio on a 2010 SESG is 2.125 (68/32), the trans ratio in 6th is 1.0, the rear tire diameter is 26.18". If you just plug those numbers into the calculator you come up with 3274 rpm at 120 mph. Definitely not a good number. To make the calculator work correctly for a Harley, you need to multiply the final drive (belt) ratio by the primary drive ratio to come up with what the calculator calls the final gear ratio. So instead of putting in 2.125 in that first box, use 2.875 instead. Now the rpm at 120 mph comes out at a more realistic 4430 rpm.
Primary = 46/34 or 1.353 Final Drive = 68/32 or 2.125 Total overall ratio = 1.353 x 2.125 = 2.875 in 6th gear
btw, Overall ratios for late model Touring bikes with 68 tooth rear sprockets:
1st = 9.593
2nd = 6.650
3rd = 4.938
4th = 4.000
5th = 3.407
6th = 2.875
Jerry