Assuming the odometer reads from the wheel sensors or even any other sensor within the drive train, the odometer mileage will not be correct if you vary away from OE tire size. Someone stated 2mph per 1" of tire size. My pickup is off by 7mph by going only slightly larger than OE at highway speed. There are plenty of free calculators online where you can enter your OE and actual tire sizes and get an accurate estimate of the speed / odometer variation caused by the modification. If your odometer shows you traveled 200 miles, but you actually traveled 230, then there is no way the odometer can be used for mileage calculation. I can't speak for Apple products, but if you use Android, DigiHUD pro speedometer is a good digital gps speedometer / odometer, with full features. I use the paid version (3 or 4 bucks). It also gives you the option of placing your phone on your dash, to cause a heads-up display to be reflected off of your windshield, but I have concerns with leaving a cell phone in direct sunlight for long periods.