Funny thing about speedo calculation is that you may be off by 5mph at 70, but not necessarily at 55...thats why I wrote it at differant speeds for the first guy...for you...
speedo says : new tires
20=21.5mph
35=37.5mph
45=48.3mph
55=59mph
65=69.7mph
75=80.4mph
85=91.2mph
any higher than that and I would worry about other things if your riding on 31" meats
Hope that helps...