A6 quattro temp tyre size difference

[basil31]

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

just to add to this, i have just had a roadforce hunter balance done, which said that the 2 new-ish budget tyres the previous owner fitted are up to 100g out on one side - the tyre, not the rim!
i'll be replacing 2 with some Vredestein Ultrac Vortis, but can't really afford to so all 4 at present, so in the short term plan to keep the budgets on the front as long as the Roadforce balance has balanced out the inperfections.

[basil31]

just to update. ran the 2 roadforce balanced tyres on the 19" wheels on the front - steering was perfect.
old 18" wheels with 2 other budgets on rear. slight vibration on those.
have just ordered 2 Vredestein Ultrac Vorties for the other 19's.
diff didn't explode so my original question was okay.