Congrats on your second century. I count the first since we really can't take today's technology and change the past. You used the most widely accepted measurement of distance when travelling at the time. It was a Century!

Another thing to think about. If you went back and re-rode that route from 1976 with a GPS, it was probably a century. I've mapped running routes with the USATF mapping site and ran them. The tangents and side of road changes inevitably will give me anywhere from .25 to .5 miles over a 10 mile run. I've made trips from Baton Rouge to MS and compared the odometer to what Google maps gave me and the numbers are close but not close enough for a shave.