You will need to know what your gearbox ratio and type/diameter of tires will be to hit any specific speed target. Once you have that, you can enter and play around with the variables here as it relates to your pinion/spur gear and actual motor: http://gearing.rcgearhead.com/Gearing/Browse

That being said, 15mph is glacial by modern RC standards, if your target is at least 15mph, just about any BL motor will get you that.