Update from the return. So the return trip from Beaufort was 300 miles. Avg speed of 69.7 and and mpg was 26.9. I don't think the 3mpg difference was the result of the 1mph difference but rather the gain in elevation. Once on 16 it is All up hill back to Atlanta from beaufort.
We also had ton of rain once we picked up 75n and I am impressed that it didn't hydroplane or get squirley on me at all.