A6 quattro temp tyre size difference

[stut4]

i have a 4.2 2006 quattro.
i purchased it with 19" rims. there is a slight vibration through the seat when hitting 60-75mph. this remains albeit slightly less worse when i swopped front to rear. i had a rebalance done 2 weeks back and still slight vibration.

i have cleaned and checked all suspension components and think they are okay.

i have swopped to 18" temporarily and i am unable to feel the vibration.

to narrow down the vibration to which of the 19" wheels tyres is causing it, is it safe to run 2 x 18's at the front and 2 x 19's at the rear?

my 18" tyres are 245/40/18 compared to 255/35/19 on the 19's. the circumference difference is 25mm which i believe is 1.2%. From what i have read its safe to run with up to 3% difference?

does this sound okay to do? i'll only run it for a couple of miles to work out which of the 4 19" wheels need more attention. i then plan to replace 2 of the 19" tyres as 3 of the exisitng ones are a poor brand.

many thanks

[rayyan171]

Not a staggered setup so you should be fine swapping for smaller sizes.

[Dr G]

Yes, that will be fine.

Simply going round a corner makes more of a difference in rotational speeds side to side than the 1.2% you mention.

[rayyan171]

Thinking about it now, I think you need your tracking done. A rebalancing of the wheels won't always do the trick. We do Hunter Alignment on our X5, not sure what Audi do but Hunter should be good enough.

Also rare car OP.

[stut4]

thanks all for the replies.
i had 2 of the existing wheels with the "best" tyres Hunter Roadforce balanced as i am planning on renewing the 2 others. One of the tyres is 100g heavier on one side than the other.
they have managed to balance out these annomilies.

i'll be getting 2 decent tyres on the other 2 rims. just can't really afford to replace all 4 just now.
i'll test drive the 2 x 19's which have just been Roadforce balanced with 2 of the other 18's which are better, and if not sorted will need to fund all 4.

the 2 exisitng tyres with decent tread must be the spawn of the devil to be so manufacturered so badly. i can now understand why some budgets are just not fit for purpose.

thanks for the comment re car - yep, think there were only 27 ish of that model sold/registered in the uk.
i've just spent the day servicing it and generally correcting all of the cheap fixes the previous owner hid. lovely engine. this hot weather has shown a couple of minor leaks so i've put a slightly more viscious oil 5w/40 which is kinder in 25+'c temperature.

[stut4]

2 x budgets roadforced balanced.
2 x new Vredestein Ultrac Vorti installed.
original vibration is fully gone at 60-80. thought i detected one at 85-90 when i shot over to the Autobahn last night