If you race at multiple tracks, you will notice that two tracks that both claim to be 1/2-mile bull rings will be different. It may be the physical makeup of the dirt; the corners may have a different radius at each end or even through the same corner; or the front straight may be longer than the back. Even if they are the same physical size, banking/lack of banking or type of surface will require a different chassis setup.
Beyond track design, the dirt is not the same. Characteristics such as how long it holds water, or whether or not it gets tackier as the evening goes on influences the performance of your car. In addition, the track may black up and start taking rubber and take the life right out of your tires. The same can be said for paved tracks. Whether dirt or asphalt, all tracks have their own individual idiosyncrasies, and the point is you will have to change the car if you race at different tracks.
Chassis setup aside, changing gears is generally a necessity if you run at different tracks, and might even be an advantage over the course of a season if you run the same track. For those of you who race the same track each week, you may still have to change gears based on the amount of grip the track develops from week to week. This could be based on the weather over the days prior to your next race. Lots of rain may mean the track will be heavy and require a different gear than what is currently in the car. It could be windy, and you know the track is going to dry out faster than when the air is still. You may change the power characteristics of your engine package. These are all things the savvy racers will pay attention to even before the car leaves the shop. The good news is that if you are running a Ford 9-inch rearend in your car the gear is easy to change.
Let's follow along as a group of racers change the gear in their racecar because of a change in the track due to the weather. The process is fairly straightforward, and with practice, you may find this process will work on your racer with just a few minor adjustments.