Barney,
Looks like you run a pretty big tire on a 7.80 digger, there's nothing wrong with that, just don't expect to run with the 33/16.5/15 crowd when it comes to MPH. It looks like you are spinnin a little high in the traps. It could be converter issues, camshaft design, intake/carb, high gear clutches or lots of other gremlins. I'm no converter expert buy I would try
www.selecttransmissions.com and talk to Eddie Sr./Jr. These guys are Super Comp/Gas racers that compete almost every weekend. They are in NY, a lot closer that AZ or CA. Just my two cents.