Interstate 80 has a total length of 2,899.54 miles.
13.3333
I80
I80/94 to I65
I80
Milton, PA
I80 miles 3.29 hours
That will be the I80.
If traveling by road in a northern route (I76 to I80) then Chicago would be the closest large city at the halfway mark. It taking the southern route (to avoid Chicago and tolls) (I79 to I70 to I465 to I74 to I80) then Bloomington, Il would closest to the halfway mark.
You should be able to make the Nebraska/Colorado border, near Ogallala.
Depending on which part of NY you're in, either I94 or I90 west (the two merge), stay on I90 when I94 splits again. I90 will merge with I80. Continue on I80 west when it splits from I90, then take I80 west all the way through to I76 in Nebraska. I76 will take you into Colorado. Where you want to go next depends on where in Colorado you're headed to. You can take I76 straight into the Denver Metro area. If you're going to Colorado Springs (I'm guessing there might be a chance you're PCSing to Fort Carson), you would take E470 south from I76 to I25, then you'd take I25 south.
The best way to go to Cheyenne, Wyoming from Atlanta, Georgia is to fly by airplane. If you are driving by car, take I75 north out of Atlanta, Georgia to Chattanooga, Tennessee, where you take I24, then I57, the I64 to St. Louis, Missouri. Then take I70 to Kansas City, Missouri, and I29 north to I80 in Nebraska. Go west on I80 to Cheyenne, Wyoming.
260 miles as the crow flies 310 miles via I80 or via US 6 340 miles via I86/I90