My gas mileage sucks in my Cummins...about 0 mpg. Course I don't run gas in my diesel.
As far as fuel mileage, my 1st Gen gets about 20 running 65mph which is around 2k RPM. If I bump up to 70 or 75 then my mileage drops a few MPG. From my experience, take your baseline at 65 which shoudl put you real close to 2k RMPs. From here you can guess your mileage. Every 5 MPH or 500 RPM increase results in about 1 or 2 MPG drop. AS you go the other way, your mileage will increase to a point.
Course, if you're wanting better mileage, get rid of the 35s adn run a set of 285s or something. That tire and gear combo will play havoc on the 727 especially if you tow anything.