Depending on the effect you're looking for, I would probably use a heavy, smooth vehicle like a Caddy or Lincoln and mount on the trunk lid with remote control camera. I'd drive forward to lessen the relative speed difference, using an appropriate lens (tele to speed things up and pack the bikers together in which case I'd drive faster, or wide to make it look more of an immense grouping and I'd drive slower to have their speed look faster).
If you drove just a little slower than the bunch, it'd be a good way to "introduce" each biker.
If you're going to shoot from the back of a bike at that speed, you're bound to have some wild vibes - but then that could add to the effect. I'd get out there and practice before the event.
You don't want to find out what you've got in post after the event. I'd have a friend follow behind (on a motorcycle, car, whatever) and run tests before the event.