Since you have new tires, just pick a sharp corner of one of the treads in the center and use that to measure from, it will take two to hold the tape. Then use you aluminum bars and measure like you have been. It will be a good test to see how accurate both ways are. Drill some holes out on the ends of your measuring bars so the bungie hooks are away from the tire, then measure with the tape touching each tire for a third test measure.
