Gear and Tire Applet
User Guide
The document is a user guide for the Gear and Tire Applet. The first
section describes general use of the applet and the other sections contain
more specific information about tires, transmissions, transfer cases, and
ring and pinion, The general section covers what the Gear and Tire Applet
is and what it can be used for. It also covers general information on
ratios and the input and output of the Gear and Tire Applet.
What is the Gear and Tire Applet?
The gear and tire form is designed to assist you making gear and tire
selections for your four wheel drive vehicle. Larger tires gain ground
clearance, increase maximum fording depth, and increase the angles of
departure and approach, but they often don't work well with OEM gearing.
Auto manufactures build trucks for the street, which means they select high
gearing for fast acceleration and better gas milage. When larger tires are
put on these trucks, the driver may find themselves burning through
clutches because first gear is so high. Engine performance and gas milage
may suffer because the engine is forced to work outside of it's optimal RPM
ranges. Top speed on the highway will increase, but so will low speed on
the trail.
There are two main motivations to regear, the first is to return the truck
to the manufactures operating ranges to improve engine performance and
decrease wear and tear on the clutch and transmission. The second is to
increase offroad performance with lower gearing. Lower gearing allows the
offroader to go slower through obstacles giving the driver better control
so there is less chance of vehicle damage and occupant injury. There is a
penalty for low gears on top highway cruising speed. The gear and tire form
is designed to allow the user to try different configurations and see what
performance will be like for different configurations before going through
an expensive gear swap.
General Information on Gear Ratios
One important aspect of gear ratios is numerically large gears are the low
ones. Gear ratios are normally stated in the form x.xx:1, e.g. 6.32:1. This
translates mathematically to 1/6.32, and since 1/2 is more than 1/6, 6:1 is
lower than 2:1. The ratio means that for every 6.32 revolutions from the
input the output will rotate once. This happens to be granny low first gear
on a Ford T18 transmission. When you are in first with the T18, the
engine does 6.32 revolutions for every 1 revolution the transmissions
outputs. Fourth gear on the T18 is 1.00:1, so for every revolution of the
engine, it outputs one revolution.
Input for the Gear and Tire Applet
The Gear and Tire Applet takes as input your tire size, transmission
ratios, transfer case low range, and ring and pinion.
The tire field will accept input in English or metric sizes. The entire
metric size must be input to calculate diameter, but the prefix 'P' or 'LT'
are optional. For example the tire size P225/75R15 could be entered without
the 'P'. For English sizes, the gear and tire form is only interested in
the diameter in inches. The rest is not needed, but if it is typed it will
be ignored. For example '32x11.50R15LT M+S' can be just entered 32.
Input of Transmission ratios, transfer case low range, and ring and pinion
all have the same basic form. The Gear and Tire Applet is looking for the
ratios and ratios are commonly expressed in the form x.xx:1. To save some
typing the ':1' part does not need to be entered. You can enter it if you
like but, it will be ignored. If you have tooth counts, for ring and pinion
for example, just divide the big number by the small. For example 41/10
ring and pinion translate to 4.10:1.
The user has the option to produce output in kilometers per hour or miles
per hour. No output is produced until the 'calculate' button is pressed.
This document is available by pressing the 'help' button.
Output for the Gear and Tire Applet
When the calculate button is pressed output is created in both graphical
and tabular form. The graph is a line graph with RPM on the xaxis ranging
from 0 to 4000 RPM. The yaxis is either MPH or KPH depending on user
input. MPH range from 0 to 80 MPH. KPH range from 0 to 120 KPH. Each
transmission gear is printed out as a seperate line in the graph first
being red, second blue, third green, fourth black, and fifth yellow. A
check box at the botton of the graph allows the user to "shift" the
transfer case into low range. The title of the graph gives the users tire
size and ring and pinion ratio.
The tabular output is also produced each time the calculate button is
pressed. The title on the tabular output indicates the units of output
either KPH or MPH. It also indicates the users selection of tire and ring
and pinion and allows the user to enter different RPMs to see the table at
various engine outputs. Each row of the table represents a gear of the
transmission first, second, third and so forth. The columns represent high
and low range of the transfer case.
The crawl ratio, which is often used to compare gearing set ups, is also
included in the tabular output. The crawl ratio represents compound low, or
first gear of the transmission and low range of the transfer case. For
example, many Wranglers have a first gear around 3.83:1, the transfer case
low range is 2.72:1, and the axle ring and pinion is 3.07:1. This means the
crawl ratio is 3.83*2.72*3.07 or around 32:1. Put some big tires on that
Wrangler and it will be difficult to control on a technical trail. A crawl
ratio around 60:1 is often good target.
There is more detailed help on tires, transmissions, transfer cases, and
ring and pinion, see the special sections for these topics.
Tires
As mentioned in the overview the tire input field will accept input in
English or metric sizes. The entire metric size must be input to calculate
diameter, but English sizes only require the diameter in inches to be
entered. Sorry, the Gear and Tire Applet does not support old tire sizing
conventions such has H78 etc. If you actually have tires this old, you'll
have to break out the tape measure and input the diameter in inches.
This section covers the method metric tires are translated to inches and
how tire size changes speed.
How to Convert Metric Tire Sizes
Metric tire sizes are made up of four components: prefix, width, aspect
ratio, and wheel size. It takes three of these components to calculate the
diameter of the tire. It sure is not a very straight forward sizing
convention.
The prefix is used to indicate general use of the tire. The tire input
field trys to ignore the prefix and if your prefix causes an error, just
don't input it. The Gear and Tire Applet currently will successfully ignore
'P' for passenger car and 'LT' for light truck. When in doubt, drop the
prefix.
The width simply the width of the tire in mm. For example the tire size
P215/75R15 has a width of 215mm.
The next component is the aspect ratio. The aspect ratio is the percentage
of the width that makes up the sidewall of the tire. In the above example
the aspect ratio is 75 or 75%, so the sidewall height is 75% of 215 or
161.25mm. If you want the diameter of the tire, you need to take into
acount that the diameter of the tire passes two sections of the side wall.
This means the sidewall takes up 161.25*2 or 322.5mm of the diameter.
The next component is the wheel size. In the above example the wheel size
is 15". If you convert the total sidewall into inches you get
322.5mm*0.03937in/mm or about 12.7". Add that to wheel size and you get the
diameter 15"+12.7" is 27.7". So a P215/75R15 tire has a diameter of about
27.7".
How Tire Sizes Effects Speed
The formula for circumfrence of a circle is used to calculate how tire
sizes effect speed. To calculate the circumfrence of a circle the Gear and
Tire Applet uses the classic formula 2(pi)r, or given the diameter it is
simply the diameter times pi. This means in the above example a 27.7" tire
has a circumfrence of 27.7(pi) or around 87". Every time the tire rotates
once, the vehicle travels 87". This can be converted to miles or kilometers
for calculations of speed given the number of engine revolutions per hour.
Anyway looking at the simple formulat d(pi) it is clear that a larger tire
travels further for each revolutions since it has a bigger circumfrence.
That is why when larger tires are swapped in, you go faster.
Transmissions
The Gear and Tire Applet allows you to enter up to 5 speeds and a reverse
gear ratio. If you have a 3 speed or don't have the reverse ratio, there is
no need to input the extra information. If you aren't sure what
transmission you have you can use the generalizations here to get numbers
that probably will be very accurate give or take a few KPH/MPH.
Generalizations About Manual Transmissions
Ratios for manual transmissions don't vary all that much. Most of the time
first gear is around 4:1 unless it granny low truck transmission such as
the T18, NP435, SM465, SM420 etc. These transmissions have a first
anywhere between 6.32:1 to 7.20:1. Generally, the final drive gear is
straight through or 1.00:1 coupling the input shaft directly to the main
shaft. I've never seen a 3 or 4 speed that didn't have a 1.00:1 final drive
gear. Five speed transmissions have a 1.00:1 fourth and an overdrive. The
overdrive fifth gear is typically around 0.75:1. So, when in doubt, use
4.00:1, 3.00:1, 2.00:1, 1.00:1 for a 4 speed.
Generalizations About Automatic Transmissions
The author of this document is not particularly knowledgable about
automatic transmissions, but he is still willing to make some
generalizations. Most automatics seem to have a first gear around 3:1 and
final drive gear of 1:1. Overdrive automatics tend to have 0.75:1
overdrive.
Tranfer Case
It wouldn't be four wheel drive without a transfer case. The transfer case
transfers power from the transmission to the rear and front axles.
Typically, the driver has 2 high, 4 high, neutral, and 4 low. The Gear and
Tire Applet supports the standard two speed transfer cases. If you have a
three speed transfer case, you'll have to enter the ratios separately.
High range is assumed to be 1:00:1 for 2 high and 4 high. Low range is
input by the user. Low range is typically around 2.50:1.
Ring and Pinion
The ring and pinion are located in the differential housing of the axle.
The pinion takes input from the driveshaft and turns the ring. The ring
spins the differential which turns the axle shafts. There are a fairly wide
variety of ring and pinions ratios available anywhere from 2.73:1 to 6.50:1
depending on the model of the axle. Since transmission and transfercase
ratios are fairly set, a change of ring and pinion is the easiest and least
expensive way to modify the gearing of your 4x4.
Figuring out what ratio ring and pinion you have is not always easy.
Depending on engine, transmission, and option package the manufacturer may
have used any one of several ratios. The easiest way is to locate and try
to read the tag on on the differential housing. Sometimes this tag is
located on the differential housing cover, sometimes it is not. It may be a
a code stamped in the housing that needs to be looked up in your technical
service manual.
One sure fire way of determining your ratio is openning up the axle and
taking a look. Often the ring is stamped with tooth count, ratio, and build
date. Sometimes it isn't stamped, but you can still count the teeth
yourself and divide ring teeth by pinion teeth to get the ratio.
An easier, but not as accurate method of determining your ratio is to jack
one wheel off the ground and count the number of drive shaft rotations it
takes to turn the tire one full rotation. Multiply this number by two and
that is your ratio. If you have a looking or limited slip differential, you
may find it difficult to rotate the tire by hand. If this is the case lift
both tires off the ground and support the vehicle with jack stands to count
rotations for one rotation of both tires.
One final note on ring and pinion, if you know the ratio of one axle the
other should be the same give or take a few hundredths. So if you find a
tag on the front, but nothing on the rear, go with the tag you found.
May 2, 1997
