How to Calculate a Rear Differential Gear Ratio

[buttmanroad]

Awsome. Now Try that in an old Jeep CJ.

[laptoph4x]

great video

[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.

[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]

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.

[81Josh93]

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

[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.

[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!

[btarrh]

You should perform this same illustration in a burnout!

[mherbold]

LOL! I have a burnout video that you may be interested in! Look in my YouTube channel for it.

[TheLoganator1]

Im in 8th grade and I understand, I want to take this class real bad and thanks for showing how easy this was!

[mherbold]

Good for you!

[The1234cadillac]

Helpful for me also thank you time to melt some tires

[marquillos1]

very simple 2 understand.

[crazyivandfw]

AWSOME, never knew this simple but great method :-)

[mherbold]

Spin once will work but it'd be a bit harder to tell for sure if you have 4.56 or 4.89's... :-)

[capecodcuda]

i usually spin the wheel once & count how many times the driveshaft spins...i guess 10 times is more accurate...lol....now only if i could court 10 & 50 @ the same time...lol

[dtpctc]

man i can never understand dis lol

i no the smaller the ratio [smaller the number] will give a higher speed, and a greater ratio will give u low end speed..but how do u no wut is the rite size tire to go with? and differentail to go with this wheel size?

[mherbold]

A smaller number will give you a higher top speed but a slower acceleration. A bigger number will give you a lower top speed but a faster acceleration. What differential and tire size you go with depends entirely on what you want to use the car for. A higher number will also cause the car to eat more gas at crusing speed on the highway. Basically you are trading off top speed versus fast acceleration and also fuel economy vs fuel guzzler.

[dtpctc]

o0o0 thnk u i understand now

[mrjost55]

thanks, I always wanted to know what the axle ratio meant. Great video

[javi1373]

ok so what would be a good street gear with 18'' wheels but good enough so i can race once in a while i need help with this im thinkin about going with 3.55

[mherbold]

How tall is the tire and how much RPM are you expecting when you race? Basically with higher ratio gears you are giving up fuel economy for performance.

[tomtom3245]

wooow, that's ultra short!!!

U must be getting lots of wheelspin and sideways action!

[mherbold]

4.86 gears (video said 4.89 but its really 4.86) are pretty typical in a drag-only car. Some people are running 5.13's with a taller tire. I'm not really getting any wheel spin at all with 29.5" tires unless the track is really cold.

[Cazador42265]

thanks!