Ok, just making sure there wasn't a a metric conversion somewhere in there. That is a pretty reasonable number for these engines.
Running 27" tires vs 30" tires is another 10% speed gain.
When you say it isn't happy going over about 40mph, does it run out of power or are you just not happy with the RPM it is spinning?
40 miles per hour, 4.11 gears, and 27" tall tires only maths out to 2045RPM. That is pretty low for a max.
