Gear ratio check
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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. | 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. | + | 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. | 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 | + | This is much easier if using a helper. |
==Other methods== | ==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. | |
| − | + | ||
| − | ''' | + | |
| − | + | ||
| − | 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 | + | 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:=== | ===Second method:=== | ||
| − | Another easy way to estimate the final drive ratio is to use a [http:// | + | 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 | + | *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== | ==Resources== | ||
| − | [http:// | + | [http://www.wallaceracing.com/Calculators.htm Various automotive calculators] |
[[Category:Identification and decoding]] | [[Category:Identification and decoding]] | ||
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