Read this interesting post on The NXT Step-Lego Mindstorms NXT Blog about how a change in the circumference of a Lego tire will change the distance it will travel. Asking students to build a vehicle that will move a specific distance is a common challenge in many of the tech labs that I've visited.
You can go about this two ways. One, trial and error. Or two, figure out the circumference of your tire and set your program to to rotate the wheels the needed number of times to cover the distance. Second method gives students an opportunity to apply a little math.
The article discusses varying results kids get because of small changes in the tires due to heat and weight. Be sure to check out the comments where a good discussion went on for several days.