Picking up vibrations....

Replaced my wheels a little while back. Went from the stock 17" setup on the 07 GT to the following

Size - 20" x 8.5"
Lug Count - 5
Bolt Pattern Metric - 5x114.30/5x120.00
Bolt Pattern US - 5x4.5/120
Offset - 35
Backspacing - 6.13
Center Bore - 74.10
Tire - 255/35-20

I get some pretty noticeable vibration in around 40-60 mph. I was planning on getting hubcentric wheel adapters to close that center bore gap but never did.

oh, pic for the curious

Did you make sure that the rotor retaining clips were removed?

also maybe have the balance checked.


also, could be that you put truck wheels on your car and the mustang is revolting

When you check to make sure that the Tinnermann nuts (those flat clips on the wheel studs) have been removed, do this on re-installing the wheels.

• Snug all five lug nuts by hand with the tire completely off the ground.

• Go around the bolt pattern again, torquing the nuts to something like 30 ft*lbs (not a fussy number, but you do want all of the lug nuts torqued down to the same torque here), still with the tire clear of the ground.

• Lower the car just enough to put a little weight on the tire and run through the pattern again. You may or may not have to do this step a second time, depending on whether you can get full torque without spinning the tire on he ground. Normally I take two steps anyway, sneaking up on the full torque spec rather than trying to get there in one big step from the 30 or so from the previous step.

What you're doing with the above is making the wheel more accurately find its center from the lug nut tapers, without car weight trying to slip the wheel a tiny bit off-center before the nuts are sufficiently tightened. Trust me, this can and will make a difference - so before you begrudge the extra time it takes (maybe a minute or so more per wheel), consider the possible annoyance that could happen by not doing this.


Norm

#1, what Norm said. Tighten lugs off the ground.
#2, rotate tires front-to-back and see if the vibrations change or go away.

If you are running aftermarket wheels, the cheaper ones are exactly that. Lower quality control, and could very well be the source of the vibrations. (I have a set of wheels like that from AM) Also while still off the ground, spin them and look for any noticeable runout.

20" wheels are gonna ride quite differently than 17's. The ride will be harsher and the vehicle will be more sensitive to steering input. However, you shouldn't have vibration. Do as others have suggested. Then, if vibration still present, I would jack car up and spin each wheel by hand, looking for out-of-round condition. If possible, I would use a dial indicator on an appropriate surface of the wheel to check for runout as I spun each wheel by hand.

Finally, if still vibrating, I would go back to tire shop and have them rebalance and check runout.

Good luck. Should be a relatively easy fix once you locate the problem.

A vibration in a specific speed range means the wheel is dynamically out of balance (assuming no other changes), meaning the balance weight isn't centered over the wheel/tire center of mass. You should have them balanced at a reputable shop.