Intermitant Vibration above 60mph

Notice a few days ago while on motorway there is an intermitant vibration like the engine is surging/hunting. Every 5 seconds the vibration comes then goesafter a few seconds and its like clock work by the second The vibration isnt too bad but is obvious even to the passenger as you can hear and feel it. There is no rise in revs. Below 60mph its not evident.
I have changed one tyre recently, put the spare on which is a new tyre but if balancing was an issue im sure the vibration would be constant and not every 5 seconds.

Anyone any ideas??

B3NY R - If you changed just one tyre (the spare) then the axle set has one tyre one size on one side and the other another size on the other. The diff will allow the drive shafts to rotate slowly in relation to each other when going straight to compensate.
Could your vibrations be due to the diff rotating slowly to accomodate the difference in tyre sizes and vibrating at one point during it's travels around the planetary gears?
Just a thought.

And if the new tyre is on the front switch it to the back and see if it makes any difference.

I shall change the tyres around and give it a go. Will i get the same issues on the rear? I think the rear tyres have equal wear so will put the rears on the front and fronts on the rear and see what happens. Thank you!

The point is the rear tyres must have the greatest circumferance....

Hmm. Why is that? what if the front right tyre has less than the rears and the front left has more tread that the rears but both rears have the same lol?

B3NY R said:

Hmm. Why is that? what if the front right tyre has less than the rears and the front left has more tread that the rears but both rears have the same lol?

Click to expand...

Not sure if your taking the **** or not but your not far from the facts. The sum of the circumference of the rears must be greater than the fronts but never the other way around. If they are the same then that's fine.