Gear changes in the OE rearend...
Gear changes in the OE rearend are harder than the quick-change rearend, but doing a little experimentation might be worth the effort to obtain maximum performance.
Power vs. Wheelspin
A lower gear will pull much better, but only if the engine is putting out sufficient horsepower at the low end and 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 spinning, so make sure you can tighten the car off the corners sufficiently to take advantage of the lower gear.
This is especially true for running on dry-slick tracks. 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 in 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 have their higher ratio gear in the car that will 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 go 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 touring Late Model drivers have said they often used no more than half throttle for the whole race for many of their wins on dry-slick tracks.
To calculate the drive ratio...
To calculate the drive ratio for a quick-change, divide the number of teeth in the top gear by the number of teeth in the bottom gear for the QC ratio and multiply that number by the axle ratio (ring-and-pinion ratio of the QC rearend). For OE rearends, divide the number of teeth in the ring gear by the number of teeth in the pinion gear for the drive ratio.
Running Past the PowerBand
If you go to a lower gear and put yourself way beyond the point where the motor produces good horsepower halfway down the straightaway, you've probably gone backward. Restricted motors are especially known for a dramatic drop in horsepower at the end of the powerband. Not only is the powerband running out, but also the restricted intake won't suck any more air at the higher rpm.
For tracks 31/44 mile or longer, using a higher gear might produce more top speed for a larger gain since we stay at the elevated speed longer on those tracks and acceleration is not a significant factor.
This is where thinking that higher rpm produces higher horsepower may be wrong. Torque is a factor in producing a higher top speed, and the torque curve may fall off before the horsepower curve does. Gearing for maximum torque may produce a higher top speed on longer racetracks.
Let the Stopwatch Decide
The best rule is to let the stopwatch determine the best gear. The fastest gear on the watch may not look or feel fast, but the lap times will tell the true story.
Divide the track into segments and time the car from the points on the track where the driver starts to accelerate to where the driver lifts going into the next turn. Use the same point on the racetrack for every measurement of elapsed time. Compare your times to your competitor's times to judge how each change stands up.
To calculate the final drive...
To calculate the final drive ratio (that includes the tire size) we divide the tire size by the drive ratio to get the final drive ratio. This number can be used to re-calculate the correct gear that would go with a new tire size.
Final Drive Ratio
The common interpretation of final drive ratio for a quick-change rearend is the combined ratio of the axle ratio and the QC gear ratio. We prefer to call this the drive ratio. We will use the word "final" later to include the tire size used in combination with the gear ratios. To find the drive ratio, we multiply the QC (quick-change) gear ratio by the axle ratio.
Tire size affects our true drive ratio. If we include the tire size in our final drive ratio, we will have the whole enchilada. To find this number, we divide the tire circumference by the drive ratio. If we have a 4.11 axle ratio, a 30-tooth over a 22-tooth QC ratio being 1.3636, then our drive ratio (DR) is 5.6045. If we then divide the tire circumference of 85 inches by the DR, our final drive ratio (FDR) is 15.1663.
The reason this number is significant is we might find ourselves having to use a larger tire some weekend. Suppose we arrive at the track late and all that is left in the tire truck are 87-inch or larger tires. Do we need to make a gear change? You bet. Here is why.
If we do the math in reverse and divide 87 (the new tire size) by our FDR number, we get a new gear ratio of 5.74. That is what our rear gear should be in order to keep our FDR at 15.1663. That means we need to install the closest gear to that-a 28-tooth over 20-tooth, 5.76 QC gear-to maintain our previous performance. If we don't, the car may be flat off the turns and our lap times will suffer.
Always keep a record of your FDR. This is the ratio that includes tire circumference information. You may find the right gear ratio for your track and then lose the advantage when you are forced to run tires of a different size. This is especially true when a track changes tire brands. We have seen the sizes of each brand differ by two inches or more.