My 68 has a m22w and 3:55's with stock wheels. Drove it to NY two weeks ago, 1200 mile round trip. I was at a constant 3200 rpm doing 65 with no issues, I did pull over a couple times when temp reached 220 but it was around 97 degrees outside (felt like 450 degrees inside). Still got 14 mpg. Not a trip I would do every week but it is doable.