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Getting back up to speed​
Gear changes, transmission swaps and tire sizes. All these changes can cause problems with accurate speedometer readings. These charts should help you narrow down the selection of the proper gear combination to use in the transmission and speedo cable to 'get you legal' again.

Here is how this works.

  • You need to know your rear end gear ratio. We provide all this information on identing your differential under our 'Index to Years' section.
  • You need to measure the the diameter of your tire.
  • Formula is: gear ratio X 20.2 (another magic number) and dividing by the tire diameter.
  • Look up the resulting number in the chart and see what the closest match is. Use those gear combinations and you should be 'up to speed'!
  • Example: 3.55 X 20.2 =71.71 divided by tire diameter 26" = 2.758
    After looking this up on the chart (large gears) we see that a 16 tooth tranny gear and a 44 tooth speedo gear will give us an accurate speedometer reading for this combination.
  • Yes, there is more than one 'correct' answer.
  • Enjoy!
tnew.gif
We also have a chart available which provides your gear ratio by comparing rpm's vs tire size.

Credits:

Large Speedo Gears
Multiply rear axle by 20.2 and divide by tire diameter.
Look up the result on this chart and use the closest number.
( # of teeth on gear )
Calculated RatioTransmission GearSpeedo Gear
2.061837
2.111838
2.121736
2.171839
2.181737
2.191635
2.221840
2.241738
2.251636
2.281841
2.291739
2.311637
2.331535
2.331842
2.351740
2.381638
2.391843
2.41536
2.411741
2.441639
2.441844
2.471537
2.471742
2.51435
2.51640
2.51845
2.531538
2.531743
2.561641
2.571436
2.591744
2.61539
2.621642
2.641437
2.651745
2.671540
2.691335
2.691643
2.711438
2.731541
2.751644
2.771336
2.791439
2.81542
2.811645
2.851337
2.861440
2.871543
2.921235
2.921338
2.931441
2.931544
31236
31339
31442
31545
3.071443
3.081237
3.081340
3.141444
3.151341
3.171238
3.181135
3.211445
3.231342
3.251239
3.271136
3.311343
3.331240
3.361137
3.381344
3.421241
3.451138
3.461345
3.51035
3.51242
3.551139
3.581243
3.61036
3.641140
3.671244
3.71037
3.731141
3.751245
3.81038
3.821142
3.89935
3.91039
3.911143
4936
41040
41144
4.091145
4.11041
4.11937
4.21042
4.22938
4.31043
4.33939
4.38835
4.41044
4.44940
4.5836
4.51045
4.56941
4.62837
4.67942
4.75838
4.78943
4.88839
4.89944
5735
5840
5945
5.12841
5.14736
5.25842
5.29737
5.38843
5.43738
5.5844
5.57739
5.62845
5.71740
5.86741
6742
6.14743
6.29744
6.43745
Small Speedo Gears
Multiply rear axle by 20.2 and divide by tire diameter.
Look up the result on this chart and use the closest number.
( # of teeth on gear )
Calculated RatioTransmission GearSpeedo Gear
0.941817
1.001717
1.001818
1.061617
1.061718
1.061819
1.111820
1.121618
1.121719
1.131517
1.171821
1.181720
1.191619
1.201518
1.211417
1.221822
1.241721
1.251620
1.271519
1.281823
1.291418
1.291722
1.311317
1.311621
1.331520
1.331824
1.351723
1.361419
1.381318
1.381622
1.401521
1.411724
1.421217
1.431420
1.441623
1.461319
1.471522
1.501218
1.501421
1.501624
1.531523
1.541320
1.551117
1.571422
1.581219
1.601524
1.621321
1.641118
1.641423
1.671220
1.691322
1.701017
1.711424
1.731119
1.751221
1.771323
1.801018
1.821120
1.831222
1.851324
1.89917
1.901019
1.911121
1.921223
2.00918
2.001020
2.001122
2.001224
2.091123
2.101021
2.11919
2.12817
2.181124
2.201022
2.22920
2.25818
2.301023
2.33921
2.38819
2.401024
2.43717
2.44922
2.50820
2.56923
2.57718
2.62821
2.67924
2.71719
2.75822
2.86720
2.88823
3.00721
3.00824
3.14722
3.29723
3.43724

Information credit:
I recently found this chart that I had typed up from 'somewhere' several years ago. The only reference I have to the chart's creator is the name Mr. P Coxe. I do not know the original source, but I do remember typing it all into an excel spreadsheet so I could use it in my shop. I present it for your use. Seeing as I did the typing a long time ago and have no way to confirm the accuracy anymore, some errors may be in here that the original creator did not make. e.g., my mistake, not his.... ok?

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Electronic pages by Al
Last Updated on 10/18/97
By Al McKenzie
 
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