After tuning the car up last night, I took the car out. I brought the car up to highway speeds, 70-80 mph, and I seem to feel a vibration, it's not all the time and there doesn't seem to be a pattern to when it happens. When accelerating its not there, sometimes during decelerating it happens, and usually stays somewhat constant on maintaining a given speed. I sort of feel it in the floor and the seat, not in the steering wheel.

Now the tires are a few years old, but haven't sat long enough to develop flat spots. The drive shaft seems tight, can't find anything apparent. Are there simple things I can check? The wheels were balanced perfectly 2 years ago, no weights are missing or anything. Again, only happens at higher speeds.

 

I don't know if this is your issue but it's worth a look. A friend of mine was talking about the exact same issue with his 68 mustang fastback yesterday. The suspension was all good so he figured it had to be tires but they all looked good and had nice tread. Anyway while washing his tires and rims he found a small gash BEHIND the rear tire. You had to move your hand in a certain way to feel it...pulled the tire and found he could put a small screwdriver halfway in...overinflated the tire and 3 more appeared. Crazy thing is he had looked at all the tires when they were out to check the issue and found nothing a few weeks back.

 

You could have a brake hanging up. When you notice it happening stop and see if you have a hot wheel.

 

X2 on having a good look at the tires, start with the easiest checks, try jacking up the car and spinnining the wheels to see how round they are, if all seems ok take the wheels for balancing, if one of them needs significant amounts of weights try swapping it for the spare to see if it helps.
Feeling a shimmy through the floor is a common symptom of a tire going bad although it can be caused by other problems.
You say the tires are some years old, anything over 5 years and I suggest you have them checked very thoroughly. If one tire is bad and the others are the same age I would budget for changing them soon, they are your first line of defense in safety.

Roger.

 

I would check the tire balance, normally the vibration will get more pronounced as you speed up!

 

I'll look, I did check the pressures, all are at 30psi. I'll have a look.

Interesting. I'll do that. Thanks.

Thanks. I'll check the brakes and then try spinning them to look for problems. I know the tires are older, but the tread is still decent, and I've looked over them carefully for cracks and such, and didn't see any.

I can't confirm this, it seems erratic. My speedometer is about 10-12 mph off at highway speeds, so when my speedo reads between 80-90 is starts, so approx 75 mph. I pushed it to 100 on the speedometer, I didn't notice it change, and I didn't want to push the old car any faster than that. Car hasn't seen speeds in excess of 50mph for over a year now.