I bought a used 99 miata, which develops a vibration at about 60mph becoming stronger when going faster (80mph +). When I take it out of gear at that speed, the vibration remains. I had all wheels balanced but it still does it. It's very low mileage (30k) and other than that in very good condition.

 

Pretty common Miata problem, from what I've read. Google "Miata shimmy", "Miata 65mph" or similar and you will see lots of threads on it.

My '99 had the shimmy, and I managed to fix it by swapping lug nuts around. I had one lug that was larger than the others on each wheel, so I moved it so the rears had 4 equal ones, and the fronts had two short and two longs each, on opposite sides of the wheel. So you might look to see if any of your lugs are mismatched. That fixed it for me until I got my 15x9s and new tires, now it's back.

 

How old is your suspension? It could just be totally bagged and it's time to get something new.

 

Did you get it road force balanced? That seemed to fix it for me.

 

I had a warped rear rotor which caused the same problem as OP describes. The 65 shimmy goes away over 65. The warped rotor just got worse over 65.

 

A bent wheel can "balance out" but still cause vibration.

But why do I have the feeling you are here to promote some shitty craigslist search ****? GTFO.

 

+1 This, or an out of round tire can likewise be balanced but not be "true". Put the car on jackstands and bring the rear wheels up to speed and see if they generate any instability or harmonics. You can also let it idle in gear while on jackstands and look for wheel and tire warpage as they spin. You can also swap the fronts and rears to check the others. It might also be interesting to note if a change is evident when driving after swapping the fronts and rears.

 

This is how I found my warped rear brake rotor.

 

i have a 99 with the same problem, i have a hardtop and it minimizes the shimmy feeling but its def still there unless i accel past that point or stay below it. some people have solved the problems with frame rails and things that help stiffen the car up.

 

I noticed that right away, he's being watched.
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you guys should only allow outside links in sigs for suppprting members

 

It's sounds like a Mazda issue because my MPV has had this shimmy issue at highway speeds since it was new.