Step 1 would be to get the ODO reading right with the correct pinion gear in the trans. I thought GPS's were dead nuts accurate but Kunkel knows more about this than I do. According to the GPS the speedo is reading 11% fast (70 divided by 63) so you would want a speedo gear with ~11% more teeth. I would double check by measuring rollout by putting a rear tire valve stem at 6 0'clock & making a mark on the pavement at that point then rolling the car forward 1 revolution till the stem is back at 6 0'clock & with your tape measure that distance (rollout) then divide Pi (3.1416) into rollout to get actual diameter then going to the MP chart in the archives along with the rear gears/dia so see if their recommended pinion tooth # matches. When you get the ODO confirmed accurate then see if the speedo is accurate either by GPS or a roadside digital MPH sign. If the odo is accurate but the speedo ain't then the speedo head needs to come out & go to a speedo specialty shop to be recalibrated but one thing at a time.