Tacoma 98 highest effeicency speed

I just finished an almost coast to coast drive from San Diego to a small town in Michgan. It was a long drive to say the least. I keep an accurate tally of my gas milage with excel. I got way better gas milage on the trip, when i was driving 65-95 mph. Im not exacly sure what speed, i know a function of RMP, i was getting the best gas milage. I lack an RPM guage in my 98 tacoma 2 wheel drive truck.
I know that theres got to be some median point where the air drag and the RPMs come to an optimum efficiency for miles per gallon.
Anyone with any input is helpfull.