I have done it once on basically a flatland trip @55mph, but it ain't easy.

 

I'd beleive the OP...when my truck was brand new, I had a weekly commute to and from college of about 150 miles...and the times of day I drove was early or late and had VERY little traffic to slow me down, make me pass, etc...was able to keep the cruise set and not touch it, etc. And I got about that...stock 245 tires and everything too though.