How to Calculate a Rear Differential Gear Ratio

This video shows how I calculated the gear ratio installed in the rear differential of my 1973 Duster 340.

Come read my blog at http://www.HerboldRacing.com

Category:

Autos & Vehicles

Tags:

• 1973
• Plymouth
• Duster
• 340
• Drag
• Race
• Car
• Rear
• Gear
• Ratio
• Differential
• Herbold
• Racing

[5148353229]

To get the exact gear ratio, take off the diff cover and count the number of gear teeth on the crown and divide it by the number of gear teeth on the pinion! At same time you can inspect your diff.

[mherbold]

@5148353229 The differential cover for some cars (like my Mopar 8 3/4" shown in the video) are permanently welded, so opening up the differential is more involved that just popping open a cover.

[81Josh93]

Hey, I tried this on my 1970 Firebird with an open differential. For every one turn of my tire I got 1.4 turns on the driveshaft and I doubt that my ratio is 1.4:1. Did I do something wrong?

[mherbold]

@81Josh93 With an open differential you need to stop the other tire from turning. Sit the other tire on some blocks or something. Then you need to turn the free tire two times instead of just one time. How much your driveshaft rotates is your ratio. For example if you follow the instructions and rotate the tire twice and the driveshaft rotates about 3-3/4 times then you've got a 3.73 gear ratio.

[FoxSkinnyRocker]

I have had this explained to be and have explained to others, but this video is the best thing I ever saw regarding this subject

[mherbold]

@FoxSkinnyRocker Thanks!

[marquillos1]

very simple 2 understand.

[buttmanroad]

Awsome. Now Try that in an old Jeep CJ.

[laptoph4x]

great video

[tiagoblanquet]

@81Josh93 Iv´e tryed this to on my 325i Bmw and i get 1.5turns on the shaft for every turn on the wheel. I have an open diff, and had the oposite wheel locked. The diff is 3.15 so i don´t get it.

[henkkaj73]

I had no idea that it can be determined with such an easy method. Got a 12-bolt Chevelle diff bolted on my pro street -65 El Camino by a previous owner and was wondering how to determine the gear ratio. Thanks a lot! :-)

[81Josh93]

@mherbold Ok thank you very much. I'll try that.