Well I found out that running 14" tires on a car that was built for 15" causes the speedometer and odometer to error. The tire size is programed into the cars processors. So my speed and mileage are off by about 7%.
You are leaving out what size the Tire is that is installed on the 14" rim. Without knowing that and without knowing a few other factors you are possibly incorrect about the 7% difference.
