HI all... Listen, I had my stock 3:55 gears swapped out for a new set of 4:56 gears due to past towing problems and the power loss from running 32x11.5x15 tires. My trucks a 94 F150 Flareside extended cab 4x2. It also has a 6"lift...

Anyway, The gear change was about 580 miles ago and I've yet to be impressed and/or atleast "happy" with the new performance.
So, I finally decided to jack up the 'ol rear end and mark the drive shaft and tire and spin the tire 1 revolution while counting the drive shaft revolutions. (the old way of checking a gear size)

Anyway, with the tire size i'm running, I figured with the 4:56 gears, the drive shaft should atleast turn 3.5 turns.....(if not 4 full turns)...

My shaft only turns 2.15 turns!! That tells me that the swap never happened, right? Cuz 3:55 gears with 32" tires, would be reduced to almost a 2:23 or so, right? It appears as though my stock 3:55 gears are still in their after I payed big bucks to have the gear change..
What do you all think?
If I have a different shop pull off the pumpkin, can they tell by looking at the gears just exactly what they really are?
I want some more solid proof before I go back to the original shop and make any accusations...
Help please....
The Drive shaft/Tire spin method is fairly accurate, is it not?
Thanks..

 

If you mark the tire and the driveshaft rotate the tire 1 full rotation and see how many turns the driveshaft makes. That will give you your ratio and takes tire size out of the equation. Another way to tell for sure what gears you have is to pull the gear cover off. Stamped on the side of the ring gear there should be the number of teeth on the ring gear/pinion. Divide the ring gear teeth (larger number) by the pinion gear teeth (smaller number) and that will tell you the ratio. That would be the best way since you will be looking at the gears and will know FOR SURE what you have. If its not right take your recipt in and tell the shop to look at the gear teeth numbers stamped on the ring gear and they should take care of it. If not thats a law suit waiting to happen (if they don't refund your money or do the gear swap).

 

The tire spin method is 100% accurate but you have to be able to accurately measure how many times the driveshaft spun. I set the driveshaft yoke so 1 of the u-joint cap bolts is at the 12:00 position. It helps to put a piece of tape on the bolt or mark it with white out or a yellow paint pen. Then while spinning both rear tires 1 full rotation I count the yoke rotations using the one u-joint cap bolt as a marker. The u-joint cap bolt should go passed the 12:00 position 4 1/2 times to give you 4.56:1 gears. The other thing you have to make sure of is to spin both rear wheels together at exactly the same time. If one turns slightly less than the other it will cut down on the rotations of the driveshaft which will give you a false reading. This applies only to an open rear diff. If your diff is fully locked then you can spin 1 wheel and get an accurate count. Even with a limited slip diff you should spin both tires together to make sure you have an accurate reading. You can also put 1 wheel on the ground and just spin 1 side to get an accurate reading.
You can also pull the cover and count the teeth on the ring gear and on the pinion. Divide the ring gear teeth by the number of teeth on the pinion and this will give you an accurate reading.

 

Looks like I type slower than Powerstroke

 

Guess something good has come of me being bored out of my mind and doing nothing but hanging around the boards. My typing skills have improved some


IMO the best way (not necessarilly the easiet or cleanest) would be to pull the cover. Look for the stamping on the side of the ring gear listing the tooth count. This way there is no variation that could come from having the tires not turn right. Luckily for you your rear axle uses 80W90 oil which is dirt cheap compared to the 75w140 synth that the 97+ F150s have to use so it won't cost you a fortune to refill the diff if you pull the cover.

 

Good grief. Put ONE rear tire on the ground (unless you have a locker). Mark the drive shaft and one tire. Spin the drive shaft 4.5 turns, and the tire will have gone 1 turn.

If you have both tires up one will spin one way and one the other. The rotations covered by one tire will be half as much (hence the near 2.25 reading!).

 

Thanks...

I did lift both tires off the ground, but I spun only i1 wheel (the power wheel) and the other tire did not move..... It moved only if I turned the power wheel real fast and hard... Otherwise, it did NOT move...

Is there something else maybe wrong??

I'm telling ya, my truck does not act any differently than when it had the 3:55's...
Heck, my speedo isnt even off......
Whats up??

Thanks.

 

HI!... The only fool proof way is too pull the diff cover off and count all the ring gear teeth. Then count all the pinnion gear teeth. Then divide the ring gear gear teeth by pinnion gear teeth and that will get you your gear ratio.

 

I used to do this all the time on my old hot rods. Jack the car up in neutral and mark a wheel or both wheels if you like. Turn the Driveshaft (not the wheels) and you can count how many driveshaft turns for one wheel turn. Its fool proof.

Do not take the cover off cause this could lead to problems with the shop acusing you of something.

 

Thanks.. But, will the amount of turns equal 1/2 of what the gear ratio is? Or will it represent the full actual ratio.. ie:4.5 turns.. (as oppossed to 2.25)

 

You are trying to determine if your stock 3:55 were replaced with 4:56 right?

For one revolution of your tire (regardless of size) your drive shaft should turn aprx 4 1/2 times for 4:56 and 3 1/2 times for 3:55. Most people think there is more to it than that, but in realty its pretty basic.