Tire size calculator This handy to see how your tires affect speedo What is the standart 14" Maverick tire
My '77 has the old alpha-numeric size on the tire pressure label of C78-14 or D78-14. This conversion chart shows that equals a 185/75-14. http://www.tirerack.com/tires/tiretech/45_conversionchart.html I don't know if other years had different factory sizes though.
Your car came with H78/14s on it. That was what was on my 72. My 69.5 had C78/14s on it. Hope that helps.
Remember that with an automatic transmission your speed is dependent on load. If you are climbing a hill then your speed will be lower compared to RPM than if you are coasting down a hill at the same RPM. RPM and speed can be off by as much as 50% with an automatic transmission but it is generally less than 1500 RPM - at any given speed.