Mazda Repair: 99 mazda miata, steering wheel, bulge

Question
I have a vibration @ 50 to 60 kms. per hr. in 4th gear. Steering wheel does not shake.. Seems to be rear R.H. wheel? What could it be? Thank-you.

Answer
could be a number of things

start small a bad or improperly inflated tire
(look for a bulge in the tire you may have to feel around the whole tire sometimes the innerbelts let go

a lose tire the nuts not properly torqued

or supsention problems