In this exaggerated example,...
In this exaggerated example, we can see where the use of a lower 5.61 gear over a 5.48 gear might improve acceleration while showing the same rpm at the end of the straightaway. Looking at 6,800 rpm, we see where we reached that speed approximately 0.8 seconds sooner with the 5.61 gear. The real gains will be smaller, but still significant. Notice how the 5.61 speed gain trails off towards the end of the straightaway but is still higher than with the 5.48 gear at 7,200 rpm.
If we chose a gear that will pull from say 5,000 rpm just past mid-turn to 7,100 rpm at the end of the straightaway, we might feel we have the right gear if the horsepower curve goals are met. That means our engine hp curve comes on at 5,000 rpm and it peaks at 7,100 rpm. It sounds like we have the right gear, but maybe not.
On some tracks where acceleration off the corners is critical for passing, a lower gear (higher ratio) might help us get off the turns better while not necessarily hurting us at the other end. The lower gear will accelerate the car quicker and if we don't lose traction with the switch, at least to the flag stand, we'll be better off.
For the last half of the straightaway, if we have gained a half a car length by now, our speed will be mostly peaked while our competitor will still be accelerating. We'll still be pulling away from the other car because the other car has not yet reached top speed/rpm.
Our turn entry will be much smoother because we will have reduced our acceleration before we lift to brake while the other car is still accelerating when the transition to braking occurs. It's very disruptive to make the change from hard acceleration to hard braking. It's much smoother to transition from steady speed to hard braking. Just ask any high performance driving instructor and he or she will agree.
Gear changes with the OEM-type...
Gear changes with the OEM-type of rearend are harder than the quick change rearend, but doing a little experimentation might be well worth the effort to obtain maximum performance.
So, we might be better off to install a lower gear, beat our competition off the corner and have a better corner entry all by doing a little experimentation. All it takes is a little effort and testing with a stop watch and the results can be evaluated. Most teams don't know what areas to test at a test session. This is one of those areas that may well improve your performance. Just work your stop watch from the mid-turn to the lift point at the end of the straight.
A lower gear will pull much better, but only if the engine is putting out sufficient horsepower at the low end, and if you can get the pulling power to the racetrack. Being able to pull great off the corner is all lost if the wheels end up slipping. So, make sure you can tighten the car off the corners sufficiently to take advantage of the lower gear.
Some teams label their gear...
Some teams label their gear sets clearly so that when it comes time to make a change, any crewmember can easily find and produce the needed gear set.
This is true especially for dirt racers running on dry, slick tracks. But for most dirt tracks the condition of the track is constantly changing. If we know the track is going to have much more bite early in the event, during qualifying and on into the early heat races, we might be able to use that to our advantage when selecting gears.
When the track is tight, we can utilize a lower gear since we will have more traction available. If the other cars still have their higher ratio gear in the car that work best under the dry slick conditions that will come later on in the evening, we might qualify better and run better in the heat race. That will get us a much better starting position for the main event.
If the track has gone dry and slick, we can then make a gear change to a higher gear that is less prone to spinning the tires to maintain grip off the corners. Power availability at the other end of the straightaway becomes less important on dry slick tracks. Many top Late Model touring drivers have been heard to say they often used no more than half throttle for the whole race for many of their wins on dry slick tracks.