First, you should verify speed with a GPS (most phones have a GPS speedo app for free) Next, with those 3.73 gears you probably are in 3rd by 20 mph or so. Lastly, changing to the ST300 won't change your highway RPM's, all final drives regardless of tranny are 1:1 unless you have an overdrive set-up. Sounds like you may want to go with a less agressive rearend, maybe a 3.08 or 3.23, that would help with the shifting issue as well as decrease your highway RPM's...just some food for thought.