Your tach is right. If it was "off" it would be off by a big factor (ie. configured for a different # of cylinders). As others have suggested, your speed might not be accurate. So you might be going faster than 60mph at that given RPM.
Assuming your 4th gear is a 1:1 ratio we'd be looking at the following:
RPM: 3100
Tires: 28.7" tall
Speed: 60mph
= Rear gears of: 4.41
Since they didn't make that gearset, my guess is you likely have a 4.11, and your MPH is off a bit.
If we set your rear gears at 4.11 instead, then at 3100 RPM you're actually going 64.43 MPH and not 60.
That's my guess anyway
.
Nothing to be concerned about really, but if you want to turn less RPM on the highway you might consider a gear change or an overdrive transmission. My vote is for an overdrive. You'll drop your RPMs ~30% doing that, versus gear changes where you'll go down about 300rpm going to 3.73. A 30% overdrive transmission you could keep your 4.11 gears and shave off over 900RPM at that same 64mph speed.
Hope this helps.