Maintaining performance like...
Maintaining performance like this means everything on the racecar has to work in unison. Keeping up on the changing needs of the final gear ratio means you will more correctly match the power of the engine to the needs of the car and the track. It is all about observation of the world around you and understanding how the performance of the car interacts with those variables.
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.
 Jack up the car and use a...  Jack up the car and use a good set of jackstands to support the chassis. You may have to use another jackstand to support the car to remove the load off the suspension. Do not use a floor jack alone to support the car during this operation. Take the time to properly support the car since you will be under it for the majority of time during the gear change. |  Take the time to sweep under...  Take the time to sweep under the car if you have not done this previously. This will get all the loose dirt and chips off the shop floor. This is very helpful if you are going to be using a creeper while you are under the car. No need to struggle with the wheels on the creeper getting fouled by a stone or random chunk of dirt. |  Use a clean bucket or suitable...  Use a clean bucket or suitable container to collect the oil. Take care to first clean any loose dirt or track grime away from the drain plug so you will not contaminate the gear oil should you plan on using it again. Reinstall the drain plug once all the oil has drained out. This will keep oil from dripping in your face while you are working under the car. |
 While one team member is draining...  While one team member is draining the oil, another can be removing the axles from both sides of the rearend assembly. |  This car has a floater rearend;...  This car has a floater rearend; if you do not have this setup, you will have to pull the brakes to remove the axles. No matter what setup you have, this is something that can be accomplished in parallel with the oil removal. |  Next, the J-bar mounting hardware...  Next, the J-bar mounting hardware needs to be removed, and the J-bar moved up and tied out of the way. |
 A zip-tie works great for...  A zip-tie works great for this application. This will keep the J-bar from interfering in the later steps. |  Remove the U-bolts that are...  Remove the U-bolts that are securing the universal joint to the pinion. |  Once the universal joint and...  Once the universal joint and driveshaft are removed use some duct tape to help retain the bearing caps on the universal joint. Then, using zip-ties again, secure the driveshaft to the chassis to keep it out of the way. You do not need to remove the driveshaft from the transmission. |