Vibration at 60mph +

[GJUK]

Hi all.

I have a 68 dart with a 400 BB in it.
I've noticed that I get quite a bit of vibration over 60-70mph or so, which at that point im in 3rd gear at 4k rpm ish.

Any ideas on what I should be looking for. The front tyres are new and balanced, the rears - not sure on.

Propshaft maybe?

Thanks

Jon

 

[4spdragtop]

Try getting up to vibration speed and put car in neutral. If problem is still there I would think suspension. If vibe lessens with car in neutral at that speed, then could be a few different issues.
Motor mount?
Transmission mount??
I would definitely get the rear tires checked and balanced.

 

[jimjimjimmy]

if you have slotted holes in your rims like cragers this could be it there is no way i can get the vib out of my car with the cragers. maybe an on car ballance . it is a minor vib but it is there and starts at 65 -70 mph .

 

[GJUK]

Try getting up to vibration speed and put car in neutral. If problem is still there I would think suspension. If vibe lessens with car in neutral at that speed, then could be a few different issues.
Motor mount?
Transmission mount??
I would definitely get the rear tires checked and balanced.

Is that okay to do with a 727 gearbox, 60 mph in D, then flick to N.... Then back to D still going 50-60mph?

I know it's okay in a modern auto...

Thanks,

Jon