Editing Gear ratio check

==Overview==
Methods to determine the gear ratio of the differential

==Codes==
Axle codes are helpful for showing what was originally installed in a vehicle. But be aware the ratio could have been changed, so the best way is to physically verify.

===RPO codes===
Most later model vehicles will have an RPO sticker somewhere in the vehicle. Some common places are under the trunk lid, in the glovebox, or in the console.

Look for codes having "F", "G" or "H" prefixes. More codes '''[http://www.crankshaftcoalition.com/wiki/GM_RPO_build_codes#MORE_RPO_CODES: here]'''.

===Axle codes===
GM axles are stamped on the axle tube (and sometimes the center section) with codes for the year and ratio. Sometimes the casting numbers can also be used to show whether an axle housing is a limited slip differential (LSD) or not.

Some sites with ratio and axle code info are:
*[http://www.novaresource.org/axle.htm Axle Codes] from novaresource.org
*[http://www.thecamaro.com/Decoding/Chevy-Axle-Rear-End-Decoding/ Chevrolet Rear Axle Code Identification and Decoding] from thecamaro.com 
*[http://www.thecamaro.com/Decoding/Chevy-Axle-Rear-End-Decoding/Chevy-Full-Size-Rear-Axle-Codes.php Chevrolet Full Size and Impala Rear End/Axle Assembly Codes] from from thecamaro.com
*[http://tech.oldsgmail.com/axle_useage.php GM A-body codes] from oldsgmail.com
*[http://www.gnttype.org/numbers/axcodes.html Gear ratio info] from gnttype.org

Ford rear ends sometimes had tags under one of the carrier bolts that had the ratio and a code for being a LSD or not. Ford also used an axle code on the body tag on some models.

==Counting the teeth on ring and pinion==
[[File:Remove rear diff cover.jpg]]<br style="clear:both"/>
If there is a removable cover on the differential, removing it and counting the teeth on the gears is the most reliable method. Count the teeth on the ring gear and the pinion gear, then divide the ring gear tooth number by the pinion tooth number to find the ratio. 
*For example:

41 divided by 10 = a 4.10 ratio

40 divided by 13 = 3.08 

On some ring gears, there will be a pair of numbers stamped into the outer circumference that will be the tooth count. It may look like 41:10 or 41-10, etc. 
[[File:45-11 ring gear stamp.jpg|border|left]]<br style="clear:both"/>
Ring gear stamped with tooth count: 45-11 = 4.09:1 ratio, for a Dana 44 differential.

==Rotating the tires method==
With a standard open (non posi traction) differential raise one wheel off of the ground, mark the tire at the 6:00 position, mark the drive shaft. Turn the tire exactly 2 full turns. Count the number of times that the drive shaft turns. 

2-3/4 turns of the drive shaft = 2.73, just past 3 turns = 3.08, 3-1/3 = 3.23 or 3.31, almost 3-1/2 = 3.42, just past 3-1/2 = 3.55, 3-3/4 = 3.73, just past 4 = 4.10 or 4.11.

With a good solid limited slip differential (posi) raise both wheels off of the ground and turn both wheels exactly 1 full turn. Everything else is the same as above. You can also do this with a standard differential as long as both wheels turn exactly 1 full turn.

This is much easier if using a helper.

==Other methods==
{{Note1}}The following methods require the drive tire diameter to be known. There are various places online that can estimate the tire diameter from the tire's size. This will be close enough.

Or, the tire can be measured by running a string around the tire, or marking the sidewall and floor, then move the car ahead one revolution and mark the ground a second time. Then simply measure the distance between the marks for the circumference. A [http://math.about.com/library/blcirclecalculator.htm calculator] can give a quick answer for the diameter. To find the diameter from the circumference long hand, use the formula: '''D equals C divided by Pi''', where '''D''' = diameter, '''C''' = circumference and '''Pi''' = 3.1416.

A table with some common tire sizes and diameters can be seen below.

===First method:=== 
# Drive the vehicle at a steady speed. Note the RPM.
##For example, if the vehicle is driven at 50 mph, and at that speed the tachometer reads 2,000 RPM, record that the engine speed was 2,000 RPM at 50 mph.
# Measure a drive tire diameter in inches.
# Multiply the RPM by the diameter of the tire.
##For example, if the diameter of the tire was 30 inches, 30 multiplied by 2,000 equals 60,000.
# Multiply the speed at which the vehicle was driven by 336.
In this example, the vehicle was driven at 50 mph. So, 50 is multiplied by 336 to equal 16,800.
# Divide the figure produced in Step 3 by the number produced in Step 4 to determine the rear end gear ratio. In this example, 60,000 is divided by 16,800 to equal 3.57. Therefore, the rear end gear ratio is 3.57:1.

===Second method:=== 
Another easy way to estimate the final drive ratio is to use a [http://www.wallaceracing.com/calc-rgr.php '''gear ratio calculator''']. 

;You need to know<nowiki>:</nowiki> 
*rear tire diameter (see details on this under '''Other methods''', above)
*transmission ratio used for speed/RPM
*and you need a tachometer 

*Drive the vehicle at 50 MPH (as an example), note and enter the RPM that was seen at the MPH that was used, then enter the rear tire diameter and transmission ratio. When you hit '''calculate''', the rear gear ratio estimate will be the result.

==Tire size table==
<table class="techrows" style="font-size:10px" border="1" cellpadding="2" cellspacing="0" width="760"><tr valign="middle"><th class="titleb" rowspan="2">Approx. Tire Diameter<br>(in Inches)</th><th class="titlec" colspan="4" width="412"><strong>P-Metric and European Metric</strong></th><th class="titlec" colspan="4" width="412"><strong>LT-Metric</strong></th><th class="titled" colspan="2"><strong>Light Truck</strong></th></tr><tr valign="middle"><th class="titleb">82-series and 80-series</th><th class="titleb">75-series and 70-series</th><th class="titleb">65-series and 60-series</th><th class="titleb">55-series and lower<br></th><th class="titleb">85-series and 80-series</th><th class="titleb">75-series and 70-series</th><th class="titleb">65-series and 60-series</th><th class="titleb">55-series and lower<br></th><th class="titleb">Flotation</th><th class="title" style="font-size:10px">Numeric</th></tr><tr><td><strong>42</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>42x14.50R17LT</td><td><br></td></tr><tr class="grey"><td rowspan="3" width="103"><strong>40</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>40x13.50R20LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>40x13.50R18LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>40x13.50R17LT</td><td><br></td></tr><tr><td><strong>38-1/2</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>38.5x14.50R20LT</td><td><br></td></tr><tr class="grey"><td rowspan="4" width="103"><strong>38</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT395/70R16</td><td>LT395/65R18</td><td><br></td><td>38x14.00R24LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>38x15.50R20LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>38x15.50R18LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>38x15.50R15LT</td><td><br></td></tr><tr><td rowspan="6" width="103"><strong>37</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT325/80R16</td><td><br></td><td><br></td><td><br></td><td>37x14.00R24LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>37x13.50R22LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>37x13.50R20LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>37x13.50R18LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>37x13.50R17LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>37x12.50R17LT</td><td><br></td></tr><tr class="grey"><td rowspan="2" width="103"><strong>36</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT375/60R18</td><td><br></td><td>36x15.50R20LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>36x15.50R15LT</td><td><br></td></tr><tr><td rowspan="2" width="103"><strong>35</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT305/70R18</td><td>LT285/65R20</td><td><br></td><td>35x13.50R20LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT285/75R18</td><td>LT325/65R18</td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="4" width="103"><strong>34-1/2</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT315/70R17</td><td>LT275/65R20</td><td><br></td><td>35X12.50R20LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT315/75R16</td><td><br></td><td><br></td><td>35x12.50R18LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>35x12.50R17LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>35X12.50R15LT</td><td><br></td></tr><tr><td><strong>34</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT305/70R17</td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td><strong>33-1/2</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT255/85R16</td><td>LT275/70R18</td><td>LT285/60R20</td><td>LT305/55R20</td><td><br></td><td><br></td></tr><tr><td rowspan="4" width="103"><strong>33</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT285/70R17</td><td>LT325/60R18</td><td>LT325/50R20</td><td>33x12.50R20LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT305/70R16</td><td>LT305/65R17</td><td>LT375/50R18</td><td>33x12.50R18LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT285/75R16</td><td>LT295/65R18</td><td>LT375/55R16</td><td>33x12.50R17LT</td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>33x15.50R15LT</td><td><br></td></tr><tr class="grey"><td rowspan="3" width="103"><strong>32-1/2</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT275/70R17</td><td>LT305/60R18</td><td>LT375/55R16</td><td>33X12.50R16.5LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT255/75R17</td><td>LT275/65R18</td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT315/70R15</td><td><br></td><td><br></td><td>33X12.50R15LT</td><td><br></td></tr><tr><td rowspan="3" width="103"><strong>32</strong></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT265/70R17</td><td><br></td><td>LT345/55R17</td><td><br></td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT245/75R17</td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT265/75R16</td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="2" width="103"><strong>31-1/2</strong></td><td><br></td><td>265/70R17</td><td>285/60R18</td><td><br></td><td>LT235/85R16</td><td>LT265/75R16</td><td><br></td><td><br></td><td>32X11.50R15LT</td><td>7.50R16LT</td></tr><tr class="grey"><td><br></td><td>265/75R16</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td rowspan="2" width="103"><strong>31</strong></td><td><br></td><td>275/70R16</td><td><br></td><td>285/55R18</td><td>LT215/85R16</td><td>LT245/70R17</td><td><br></td><td>LT345/55R16</td><td><br></td><td><br></td></tr><tr><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>LT245/75R16</td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="3" width="103"><strong>30-1/2</strong></td><td><br></td><td>265/70R16</td><td>285/60R17</td><td><br></td><td><br></td><td>LT285/70R15</td><td>LT325/60R15</td><td><br></td><td>31X10.50R16.5LT</td><td>9.50R16.5LT</td></tr><tr class="grey"><td><br></td><td>265/75R15</td><td>265/65R17</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>31X11.50R15LT</td><td><br></td></tr><tr class="grey"><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>31X10.50R15LT</td><td><br></td></tr><tr><td rowspan="2" width="103"><strong>30</strong></td><td><br></td><td>255/70R16</td><td>275/60R17</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><br></td><td><br></td><td>255/65R17</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="2" width="103"><strong>29-1/2</strong></td><td><br></td><td>245/70R16</td><td>265/60R17</td><td><br></td><td><br></td><td>LT225/75R16</td><td>LT285/60R16</td><td>LT315/55R16</td><td>30X9.50R15LT</td><td>8.75R16.5LT</td></tr><tr class="grey"><td><br></td><td>265/70R15</td><td>245/65R17</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td>7.00R15LT</td></tr><tr><td rowspan="3" width="103"><strong>29</strong></td><td>205R16</td><td>235/70R16</td><td>255/60R17</td><td>255/55R18</td><td><br></td><td>LT235/75R15</td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td>205/80R16</td><td>255/70R15</td><td>255/65R16</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><br></td><td>235/75R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="3" width="103"><strong>28-1/2</strong></td><td><br></td><td>225/70R16</td><td><br></td><td><br></td><td><br></td><td>LT225/75R15</td><td><br></td><td><br></td><td>29X9.50R15LT</td><td>8.00R16.5LT</td></tr><tr class="grey"><td><br></td><td>245/70R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td><br></td><td>225/75R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><strong>28</strong></td><td><br>215/70R16</td><td>275/60R15</td><td>255/55R17</td><td><br></td><td><br></td><td>LT245/65R15</td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="2" width="103"><strong>27-1/2</strong></td><td><br></td><td>225/70R15</td><td><br></td><td><br></td><td><br></td><td>LT215/75R15</td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td><br></td><td>215/75R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td rowspan="3" width="103"><strong>27</strong></td><td><br></td><td>205/75R15</td><td>255/60R15</td><td>255/50R17</td><td><br></td><td>LT205/75R15</td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><br></td><td><br></td><td>225/60R16</td><td>305/50R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><br></td><td><br></td><td>215/65R16</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="2" width="103"><strong>26-1/2</strong></td><td><br></td> <td class="dark">225/70R14</td><td>245/60R15</td><td>295/50R15</td><td><br></td><td>LT195/75R15</td><td><br></td><td><br></td><td>27X8.50R14LT</td><td><br></td></tr><tr class="grey"><td><br></td><td>195/75R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td rowspan="3" width="103"><strong>26</strong></td><td>185R14</td><td>195/70R15</td><td>235/60R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><br></td><td>215/70R14</td><td>215/65R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr><td><br></td><td>205/75R14</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td rowspan="2" width="103"><strong>25-1/2</strong></td><td><br></td><td>205/70R14</td><td>225/60R15</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr><tr class="grey"><td><br></td><td>195/75R14</td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td><td><br></td></tr></table>
==Resources==
[http://www.wallaceracing.com/Calculators.htm Various automotive calculators]

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