The steering wheel vibrates something awful at 45 to 55 mph, It stop vibrating after that speed or lower the 40 mph. I asked around but all I got was I need new shocks or tie rods. The left tire is wearing on one side only, but is not cupping like a shock does to a tire. I had the car inspected with car in air and the wheel does move. I'm rotating tires tires hoping this will help. Any ideas?

front wheel drive?.....if so...perhaps alignment......if not....perhaps bent frame?....i dont know much...but i do know that these are some of the problems that will cause vibrations that you didnt mention

I would start by getting the tires balanced first, all four of them. Make sure you ask them if any of them has a slight seperation on the tread also. This will cause a vibration for sure. A bent frame or a bad alignment will not cause a vibration, just a bad pull to one side.

check the u joints on your driveshaft. Balance tires first

I recently instaled a set of Ford Motorsport 3.55's. It's an 88 GT with a five speed and 104,000. The vibration starts at 70mph. It never happened with the 2.73's. Everything is tight in the rear end. There is a new pinion bering thats all. Some say an aluminum drve saft will take it right out. I say "No @#%$". The aluminum is soft and will absorb more harmonics. I need help. When I hit 70 all I think about is the damage I'm doing. Please

Funny you posted this because both myself and my brother have had 3:55 installed (but the gears were used) and a vibration started right after. I am suspecting a drive shaft out of balance. As soon as the weather warms up and I get the car back on the road I am sending out the shaft to have it checked out.
In the mean time anyone else have the same thing happen? Some possible cure's.

For the first question. Set the tire pressures and check for tread separation. If there is none, have tires rotated/balanced.

As for the second question...you see that big chunk of metal hanging off the bottom of the rearend? That's designed to absorb vibrations, but it's tuned to the harmonics of the stock gears, not lower gears. That's why you're feeling the vibrations after the gear change. You can chuck that and install an aluminum driveshaft and your vibrations will go away.

I have the exact same problem that started when I installed my 3:55's (purchased from Ford Racing). I have since installed the FMS Aluminum driveshaft, had all four tires high speed balanced at a quality performance tire shop along with front end alignment. Problem persisted. I just recently swapped out all four tires hoping that would resolve problem (needed replacement anyway). I still have the vibration around 70mph <img src=http://www.ezboard.com/intl/aenglish/images/emoticons/embarassed.gif ALT=":o"> . Thought it was specific to my mustang until I read this post. Now I am really beginning to wonder about the 3:55's.

Dude, to GT50. After the vibration started I had my drive shaft balanced. The shop said it wasn't to out. He added a little weight. Didn't do @#%$. I flushed 40 bucks down the bowl for nothing. My shaft has new u-joints also. I thought the backlash was too much. I ripped on the pinion nut to put more pre-load on it. Nothing. Now that more of you guess answered I just want to know if all of our rear ends are trash. I pray to the mustang gods that they are not.

TAKE IT OUT OF FIRST GEAR DUMMY!!!

Seriously, If all you did was change gears, then I can't see how this alone would do it. I have changed many gears in these rear ends and never had this problem.
maybe 1)pinion preload is incorrect(you said it was)
2)driveshaft was banged/weight knocked off.(you said you balanced it).
3)there was a balance problem before but you never noticed because you didn't drive at the rpm you do now(steady rpm in any gear causes vibe or just speed related?)
4)tires(why would they go out of balance during the gear change? you said you balanced them though)
5)pinion bearings loose/worn(you DID put new bearings/seals in RIGHT?)

Well when I did Jesse's gears (3:55) I use Motorsports gears. We do not have a vibration. There was an alignment mark for the pinion flange. If not aligned could cause a vibration.

I was also told to try and rotate the driveshaft on the pinion flange. There are two sets of mounting holes

I put in a new seal. I turned my drive shaft a few times. I have new tires so they are not the problem of the vibration. Some say the aluminum drive shaft will take it out, others say it didn't. I will take off the balencer on the rear end and start from there. But it's not only me with the problem. There are other guys out there too. Some loster/Chevy lovers say the rear end is junk! Is it? I pray to the 5.0 Mustang Gods that it isn't!

The rear end isn't junk! 8.8 are in rangers and halftons with different tubes/axles, and numerous others. I personaly am a 9" fan, but I drag race! My 92 halfton has one and one problem after 250,000 plus kilometers(175,000 miles or so) isn't too bad considering it was a leaking axle seal. Changing the seal though I noticed a common thing for any c-clip type rearend, the axle where the bearing rides is getting pretty grooved. But then I treat it like a 3/4 ton too!!
Bottom line, the rearend is NOT inherently weak, however, slicks and 4.56's and over 350hp in an otherwise stock rear can get rid of spider gears pretty quick.(And enough stupidity can even blow up a 9"!!!)

Just a question to anyone who looks through this topic. Do any of you guys with 3.73's have a vibration? I like the 3.55's just because of the mileage I can get on the high-way. My friend has a Ranger with 3.73's and the take off just rips you back into the seat. What are some of the RPM's you guys are getting on the high-way at 70 mph. I might just buy a 3.73 rear end and get rid of this vibrating 3.55 one. Just give me some ideas and tell me what gear you like better. Thanks!

i have 3.73 and my vibration is very bad.i rebuilt the whole frontend
trying to fix the problem and i was thinking about the driveshaft myself
now i am as confused as the rest of you.i like my 3.73 good pull.
does your vibe get worse when you let off the gas at 70 or is it constant
after 60 or so.mine gets worse after i let off.