Topography and wind makes outdoor speeds very subjective. If you could get an estimate of your FTP watts per kg you could see where you are on the power profile chart.
http://www.cyclingpeakssoftware.com/images/powerprofile_v4.gif
If you have a Kurt Kinetic Road Machine
http://www.1upusa.com/kurtkinetictrainer.htm
you can get a fairly accurate estimate from a speed to power chart like this. I road for an hour on the KKR at 19.8mph=252watts FTP. 252/89kg=2.83w/kg
17mph=183.33w
17.1mph=185.53w
17.2mph=187.74w
17.3mph=189.98w
17.4mph=192.23w
17.5mph=194.51w
17.6mph=196.80w
17.7mph=199.12w
17.8mph=201.46w
17.9mph=203.81w
18mph=206.19w
18.1mph=208.59w
18.2mph=211.01w
18.3mph=213.45w
18.4mph=215.91w
18.5mph=218.39w
18.6mph=220.89w
18.7mph=223.42w
18.8mph=225.96w
18.9mph=228.53w
19mph=231.12w
19.1mph=233.73w
19.2mph=236.36w
19.3mph=239.02w
19.4mph=241.70w
19.5mph=244.40w
19.6mph=247.12w
19.7mph=249.86w
19.8mph=252.63w......My FTP
19.9mph=255.42w
20mph=258.24w
20.1mph=261.07w
20.2mph=263.93w
20.3mph=266.81w
20.4mph=269.72w
20.5mph=272.65w
20.6mph=275.60w
20.7mph=278.58w
20.8mph=281.58w
20.9mph=284.60w
21mph=287.65w
21.1mph=290.72w
21.2mph=293.82w
21.3mph=296.94w
21.4mph=300.09w
21.5mph=303.26w
21.6mph=306.45w
21.7mph=309.67w
21.8mph=312.92w
21.9mph=316.19w
22mph=319.48w
22.1mph=322.80w
22.2mph=326.15w
22.3mph=329.52w
22.4mph=332.92w
22.5mph=336.34w
22.6mph=339.79w
22.7mph=343.26w
22.8mph=346.76w
22.9mph=350.29w
23mph=353.84w
23.1mph=357.42w
23.2mph=361.03w
23.3mph=364.66w
23.4mph=368.32w
23.5mph=372.01w
23.6mph=375.72w
23.7mph=379.46w
23.8mph=383.23w
23.9mph=387.03w
24mph=390.85w
24.1mph=394.70w
24.2mph=398.58w
24.3mph=402.48w
24.4mph=406.42w
24.5mph=410.38w
24.6mph=414.37w
24.7mph=418.39w
24.8mph=422.44w
24.9mph=426.51w
25mph=430.62w
25.1mph=434.75w......Lance Armstrong FTP maybe
25.2mph=438.91w
25.3mph=443.10w
25.4mph=447.32w
25.5mph=451.57w
25.6mph=455.85w
25.7mph=460.16w
25.8mph=464.49w
25.9mph=468.86w
26mph=473.26w