another format that could be good is a high speed GPS comp
you have a gps base camp/garage 1st gps point is given to you and on the sound of the buzzer you all leave the perimiter of the garage at what ever speed you like.
some cars may travel a short distance to there punch others a large distance but over the day it evens out
so once you have got your punch you head back to the garage and enter the garage area at 5mph hand your gps board in and pick up a new one leave the garage and then travell as fast as you like.
the good bit about it is that you will not know what punch your heading for and untill you get there.
i have competed in this type of event an the teams are just smile all day
and you have the added skill of gps which at present there arnt any other gps events?
dave w a thought brain hartly COR aways run an event on strict 15mph? hardly a speed event though i could be wrong