Thank you for your input. Someone suggested that I check the actual speed at 10 mph intervals to indicated speed by use of a GPS speedometer app, write down the correlations and calculate the ratio for each 10 mph increment. Then add the ratios together and divide by the number of ratios obtained for the average ratio. Then multiply the number of driven teeth by the ratio and it will provide the number of driven teeth required. I did this for increments from 20 to 70 mph and obtained an average ratio of 1.167. I then multiplied the number of existing teeth (18 x 1.167) and got 21 teeth. I changed from an 18 to 21 tooth driven gear and it now matches spot on with the GPS app. In this manner you do not need to know what rear gear ratio you have or tire diameter. However, knowing my rear gear ratio and tire diameter, I checked several published equations, and found two equations also yielded 21 teeth when entering the rear gear ratio and tire diameter. Questions answered and issue resolved. Thank you for responding.