Best RPM for highway driving? Tires too?

[TwoFiftyShifter]

I just installed a tach on my '87 K20, and I was wondering what the best rpm for mileage is on the highway. Now I know 1800 rpm is usually best but the highway near me is really hilly, and I need lots of throttel below 2krpm to climb hills, resulting in worse mpg. Is 2500 rpm too high for good mpg?

Also, I currently have 245/75r16 tires, and am soon due for new ones, I'm wanting something a little taller looking to fill the wheel wells just a little better, what tire size? 235/85 looks ok on other trucks... what do you think?

The truck in question is an 87 TBI350, 3.42 gears and 4 speed granny transmission, seems to run 2200rpm or so at 60mph, who knows how accurate the speedo is, thanks for the input.