Is there a way to determine distance from a zip code?

For example, we would like to point our customers to a dealer closest to them. We would like to have the customer enter in their zipcode and then we would like to display a list of dealers in their area, within 50 miles.

Is there a calculation to use, such as: the first 3 digits of the zipcode will be a 50 mile radius and the first 4 digits will be a 25 mile radius and so on?

Thanks,

Jerry