Most of these cycling computers I have seen ask you to enter a circumference for the tire and sometimes give you a table to look up the circumference for standard tires sizes. Don't trust the table values. The better way to make sure you are entering the right value is to actually measure the tire/wheel combo you have. To do this, find a flat section of pavement (outside) or floor (inside) and position the front wheel with the valve stem at the very bottom of the wheel. Mark this position on the ground or floor. Roll the bike forward in a straight line until you get the valve stem again at the very bottom of the wheel. Mark this position and measure the distance between the two marks. Convert this to millimeters and then enter the value into your cycle computer.