Tires can take a fair amount of heat and do so every time they are on the road so it's really not as big of a problem with a firebox as many think. I would look for about 6 inches of airspace between them. Once you use it a few times, measure your tire temp with a laser thermometer and make sure it's not much hotter than 175 degrees F. If it is, you can always add in a quick heat shield.