If grip is good, I would say Blue Rear and Double Blue front for 17.5. Maybe even Blue front. If it's low/med, and you can't get the rears to stay hooked up for the full 8 minutes, then Grey Rear and Black Front. At least at our track this is what works. However, we run 13.5 blinky here, so it's Green Rear and Blue front that works great.