To calculate the Drive Ratio for a quick change, divide the number of teeth in the top gea
If you go to a lower gear and put yourself way beyond the point where the motor produces good horsepower half way down the straightaway, you've probably gone backwards. Restricted motors are especially known for a dramatic drop in hp at the end of the power band. Not only is the power band running out, but also the restricted intake won't suck any more air at the higher rpm.
For the longer tracks of 3/4 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 the 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 hp curve does. Gearing for maximum torque may produce a higher top speed on longer racetracks. Many teams have been both fooled and surprised by gear changes that produced different results than were anticipated.
To calculate the Final Drive Ratio that includes the tire circumference, we divide the tir
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 exact same point on the racetrack for every measurement of elapsed time. Compare your times to your competitor's times to judge how each gear change stands up.
The common interpretation of Drive Ratio for a quick change (QC) 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 ratio by the axle ratio to find the Drive Ratio.
The tire size affects our true Drive Ratio (DR). So, if we include the tire size into our calculations, we will have the whole enchilada. To find this Final Drive Ratio (FDR) number we divide the Drive Ratio into the tire circumference. If we have a 4.11 axle ratio, a 30 tooth over a 22 tooth QC ratio being 1.364, then our DR is 5.6045. If we then divide the DR into the tire circumference of, say, 85 inches, our FDR is 15.1663. This is a number we need to record and remember.
In this example, we have a new tire size of 87 inches in circumference, up from the 85 inc
The reason this number is significant is because we might find ourselves having to use a different size tire some weekend down the road. Suppose for whatever reason 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's why.
If we do the math in reverse and divide our FDR number into 87 (the new tire size) we get a new DR of 5.74. That is what our rear gear should be in order to keep our same FDR at 15.1663. That means we need to install the closest gear to that, a 28 tooth over 20 tooth gearset equals a 5.76 QC gear to maintain our previous performance. If we don't make this change, 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 have found the right gear ratio for your track and then lose the advantage when you are forced to run different sized tires. This is especially true when a track changes tire brands where we have seen the sizes of each brand being different by two inches or more.