Everything I have read says the 48 Plymouth can standard with 15 inch tires. So when restoring the car I put 15 inch rims back on the car, now using a speedometer app on my phone I find that the speedometer has to register 80 miles to actually be going 60 miles. I can't believe that the drive gear in the transmission would make that much of a difference. I would really like to get it to register closer to the actual speed. Any ideas on how to fix or any adjustments in the speedo head that would help correct the problem. Thanks