gearing vs mpg
hmmmm.. i found a way to get the site here.
http://www.bigblockdart.com/techpages/speedtire.shtml
if i put the numbers in correctly. and with a .64 OD (closest they have on the site to your .68) looks like you may need more gear. 73 mph at 2200 rpm. mine used to really like cruising at 2400-2700 rpm if i remember correctly.