Go with the GPS. That's good that the speedo and the odo are off the same amount as that tells you the speedo magnetic gismo which is triggered by the odo is spot on. As you know just need to get the correct speedo pinion gear in the trans. If that (44 tooth) gear ain't commonly available then might need to source a speedo gear adapter which has metal gears of a different ratio in a little box that goes inline in the speedo cable to alter the ratio so you're final ratio will come in correct