Just check mine on a short 40 mile trip down the interstate. Was going steady at 70 MPH at about 2400 RPM. I have had it up to 75 in the past in Kansas on I-70. I have a 90 Regency 6CTA 8.3 Cummins with an Allison Trans 4 spd. Tires are 265-75-22.5. Also..... Just wanted to mention that I live about 7 miles from Ft. Leonard Wood, Missouri. Wish I knew you were coming. Other than my own, I have not seen another Barth up close.

Jax

Jax, with your tires and the the engine/trans combination, it is not the same and a Breakaway with 19.5 tires.

based on the data you provided the axle ratio is 3.83

Or, W&B, you can sell your Barth to someone who is ok with the set up and buy a Barth that already has a 6-speed in it. And it may not be a Breadaway. Maybe a Regency. Lots of options.
Jim