How do I figure out what rearend ratio to run?
This can be a difficult question, because this subject is much more complicated than it might seem, at first. First off, you need to establish what engine RPM is optimum, at a particular speed, for your needs. This can often be the place where most "engineers" make mistakes, often mismatching engine RPM and speed, because they "think" an engine needs to turn a particular RPM, rather than "figuring" the optimum RPM for a particular engine/vehicle combination. For a "combination use" vehicle, this is impossible to attain, and, often, leads to poor compromise. For example, some people want to use their vehicle for work commute, but they also want the vehicle to run 12 second elapsed times, at the drag strip, while maintaining excellent gas mileage; Don't do it!! If you want to run 12 second e.t.'s, get a drag car, and a trailer, and then buy a good, reliable, and ecomical, "commute" car.
Now, if you like math, pull out your calculator, and read the following chapter. If you do not, let JavaScript do the math for you by going to my JavaScript RPM/gear ratio calculator to figure your particular scenario.
To figure proper final drive ratio, based on a calculated "optimum RPM", for a typical street car, I choose to use the function: 20171.92*x/z=y where x=gear ratio, y=engine RPM's at 60mph, and z=tire diameter, in inches. This function does not take into account, torque converter slippage factor, or any other transmission ratio, such as an overdrive gear, so you will have to figure them in, based on your particular example. I usually use a number like 5% on the torque converter slippage factor, but this will vary, depending upon engine RPM (if the final drive ratio is 2.75:1, versus 4.11, the slippage factor is slightly higher, because the torque converter would be closer to "stall speed").
Let's make up a "real live" example. Suppose we have a truck, with a 4.56:1 rearend ratio, and 29" tall tires, using a typical three-speed automatic (no overdrive). We have decided, taking into account, all of the engine and vehicle variables, and demands, that the desired engine RPM's, at 60mph (we cruise at 60mph, to work everyday), just happens to be around 2700RPM's. First, it might be helpful to estimate the engine RPM figure, currently, with the 4.56 ratio:
20170.92*4.56/29=y or: y=3171 RPM's
Now, figuring in a little slippage, add another 160 RPM's (approximately 5% slippage) giving you a current estimated engine speed of 3331 RPM's. Now, it's time for some more math:
4.56/3331=x/2700 or: x=3.69
Round off the number to something that is currently available (3.73:1 might be a good choice, if it is available)
- Figuring optimum engine RPM
- Figuring optimum engine RPM, as I said before, can be quite complicated, and should not be done hastily. Induction system (carburetor, or fuel injection, intake manifold system, and intake porting), cam selection, bore/stroke ratio, rod length, compression ratio, exaust system, ignition system, all have a profound impact on "optimum engine RPM". In addition, vehicle use is very important. For example, I have a Toyota Landcruiser, that I use primarily for off road "rock crawling" (very slow 4 wheeling, over rocks). In this example, I built an engine that would have a "rock solid idle" and run strong between 500 and 2000 RPM's. To get an engine that would run well at this very low RPM, I had to sacrifice high RPM horsepower, when I chose my induction, and camshaft, but, since I only operate this vehicle in the 500-2000 RPM range, I did not care, in the slightest, how it ran at 4000 RPM's. For my use (rockcrawing), gear ratio is an easy thing to pick... I just wanted the lowest gears I could possibly get, but, I still wanted to be able to drive the vehicle to the trail, so I needed to stay "within reason". I chose 4.11 gears (because they happen to come "stock" in a Landcruiser, making parts very easy to get), but I chose to get my lower ratio by installing a 2.331:1-low-range transfercase(instantly giving me the equivelant of a 4.11x2.331, or a 9.557:1 effective ratio), and then, installing a SM465 transmission, which has a low gear ratio of 6.5:1 (give or take), which gives me a final ratio of 62.27:1, in low gear. Doing the math, I come up with 634 RPM's to go 1 MPH, which seems to do the trick.
So basically, the following function can be derived to relate X, gear
ratio to Y, engine rpms at 60mph in a 1:1 trans config (ignoring
slippage factor), and Z, tire diameter in inches:
20171.92*x/y=z
Written by Ken Bachellerie. Copyright © 1997. All rights reserved.
Do not duplicate or redistribute in any form, without permission from the author.