With the headlight cowl there isn't much room , you may want to check out a Dakota Digital tach. I had a tach on my Superglide and they are nice , but after 20k miles on it I got to know where the motor was running pretty well. On my FXDWG3 I don't really miss it .
I got this from the Nightrider site , calculated for the FXDWG3 using a 90/90x21/Fr 150/80 16 Rear 3.15 final
MPH Calculator
RPM 1 2 3 4 5
1000 7.5 10.9 15.2 19.6 23.9
1200 9.0 13.0 18.3 23.5 28.7
1400 10.5 15.2 21.3 27.4 33.5
1600 11.9 17.4 24.4 31.3 38.2
1800 13.4 19.5 27.4 35.3 43.0
2000 14.9 21.7 30.4 39.2 47.8
2200 16.4 23.9 33.5 43.1 52.6
2400 17.9 26.1 36.5 47.0 57.3
2600 19.4 28.2 39.6 50.9 62.1
2800 20.9 30.4 42.6 54.8 66.9
3000 22.4 32.6 45.7 58.8 71.7
3200 23.9 34.8 48.7 62.7 76.5
3400 25.4 36.9 51.7 66.6 81.2
3600 26.9 39.1 54.8 70.5 86.0
3800 28.4 41.3 57.8 74.4 90.8
4000 29.9 43.4 60.9 78.3 95.6
4200 31.4 45.6 63.9 82.3 100.4
4400 32.9 47.8 67.0 86.2 105.1
4600 34.3 50.0 70.0 90.1 109.9
4800 35.8 52.1 73.1 94.0 114.7
5000 37.3 54.3 76.1 97.9 119.5
5200 38.8 56.5 79.1 101.8 124.2
5400 40.3 58.6 82.2 105.8 129.0
5600 41.8 60.8 85.2 109.7 133.8
5800 43.3 63.0 88.3 113.6 138.6
6000 44.8 65.2 91.3 117.5 143.4
6200 46.3 67.3 94.4 121.4 148.1