Topic: Can anyone Identify gears sets by #of drive shaft rotation for 1 tire rotation

[Xman4]

I think my 85' 4runner my have different gearing front and rear?  the rear gets 2 full driveshaft rotations for each full tire totation. and the front gets 2 1/2 drive shaft rotations for each full tire rotaion.  Can anyone out there identify gearing by this method.

[blackdiamond]

I think that I have finally figured this problem out myself.

If you have open differentials you have to actually roll the truck with both tires on the ground and compare the drive shaft rotations to one rotation of the tire.  I don't know why, but if you have one or both of the tires off the ground it doesn't work.

If you have a limited slip or lockers, you can simply put it on stands and rotate the tires.

If your gears were as far off  as the info that you are getting your truck would pull itself apart in 4wd.

[skid]

The math seems flawed unless tires on one side of the truck are held stationary.   With the 2 and 2.5 driveshaft rotations, the rear would be 4:1 (3.90/4.10?)and the the front 5:1(4.88/5.29?)

[Xman4 [OP]]

Yeah man this thing bucks like a bronco in 4wheel drive! Obviously I shut her down when I realized there was a problem. but it would have torn itself apart if I let it.  Someone told me with open diffs you need to rotate tires twice so rear would be 4 driveshaft rotations for every 2tire rotations and front would be 5 drive shaft rotations for every 2 wheel turns.

[blackdiamond]

For 4.10 gears the driveshaft should turn 4+ times for 1 rotation of the tire.

I was trying to figure out the gears on my buddies truck and it didn't work with either one or both of the tires off the ground.  We checked my truck that has a limited slip front and locker rear using the same method and it worked great.

If it is really that far off (i.e. 1/2 turn of the drive shaft difference)  I would suspect that it wouldn't even roll very far when pushed without binding up and stopping.

[BigMike]

yes, if it is an open diff, then you either need to hold one tire still (if vented rotors, then put a thick screw driver into a vent and let it bump up against the caliper), or you'll have to multiply the ratios by two.

You should be able to identify if it is near a 4:1 or near a 5:1, but determining if it is a 3.90 or 4.11, or a 4.88 or 5.29 would not be possible I would say.

I guess the best way to do it would be, say you didn't know if it was a 3.9 or a 4.11. Then I would carefully count the numbers of rotations through 10 or 20 rotations. Then, at that number, the two ratios would begin to distance themselves and you should be able to determine the ratio by division.

Mike

[79coyotefrg]

take a screw driver and make a mark on the third member,  and the drive flange,  and mark the tire and the ground,  then jack one tire up,  in neutral,   have someone turn the tire til the mark makes 2 revolutions,   you will be watching the pinion flange carefully counting its revolutions  in 2 tire revs,  your will get 4 and just a tad more, as in 1/10th of a revolution,  about 3/4 inch past the 4th revolution,  this will be 4.10's

follow the same for the front,  rotoate the tire 2 times exactly, if you get  almost 5 revolutions  then you have 4.88's   if you get   5 revs and about a 1/4 turn then you have 5.29's

its very easy and almost perfect

i have 4.37's   and i get almost 4 1/2 revolutions, if i had just a hair over 4 and a 1/2  i would have 4.56's 

[skid]

Rotate the tire 4 times and divide by 2 may give a more accurate reading.

[79coyotefrg]

well  as long as he marks everything and pays attention he can get very accurate results with 2 turns

[Duffil]

  1.put the truck on the ground.
            2.push the truck and count revolutions, while noting tire position.
            3.enjoy knowing your gearing.

it is the easiest way to do it, no multiplication or division.

[79coyotefrg]

1.put the truck on the ground.
 2.push the truck and count revolutions, while noting tire position.
 3.enjoy knowing your gearing.

it is the easiest way to do it, no multiplication or division.

its not easy to push a truck and count the pinion revolutions at the same time

[Duffil]

as long as your spare isn't under the truck, it's fine.  push from the rear axle.

[DTB]

as long as your spare isn't under the truck, it's fine. push from the rear axle.

umm ok. Being on flat ground would be a plus too.

[skid]

While were here, lets over think this. Figure tire roll out(distance traveled in one revolution)and buy a gps and tach.  Run the truck at a given speed in 4th gear and note RPM.  Doing the math and figure the ratio.    Yes I can be a smartass.

Also,  looking at the build tags will tell you what the truck came with.  Likely this is still in one of the axles.

[79coyotefrg]

yea,  i think its extremely simple,  but oh well