I had a 5.38 in my bus, with same size tires. However, I have a 5 speed manual with a 1:1 top gear and I was seeing 2600 RPM at 55 MPH - the overdrive makes a big difference for you. I swapped my ratio to 3.91 and my top speed is now around 78 MPH, though on the freeway I'm averaging around 70 for top speed on any uphill.
I haven't double checked Booyah's math but based on my own calculations and experience, everything sounds about right. The 5.38 is a fine gear ratio for those who have overdrive gears. Not so much for those with 1:1 top gears.