A8 wheel & tyre size

[CHEF_GOLF]

I am after a little help. I am entering a charity road rally and we are customising an Audi A8.
I have bought a set of Bentley continental GT wheels. I have been informed that I will have to have different profile tyres on the fronts to rear to make the rolling surface the same so not to damage the Quattro system.
The current set up is 245/45/18 et48
New size proposed is 245/40/20 front 275/35/20 rear et41.
Question is do I need a different profile at the front? Thanks

[PositronicRay]

A useful calculator.

https://www.willtheyfit.com/

You'll need to keep the same circumference all round.

[CHEF_GOLF]

I would have thought I would have had to run the same profile all round as this would have kept the same diameter/ circumference of the wheel. So run 35s all round.
I am a little confused by all of this

[PositronicRay]

CHEF_GOLF said:

I would have thought I would have had to run the same profile all round as this would have kept the same diameter/ circumference of the wheel. So run 35s all round.
I am a little confused by all of this

The profile is the height of the sidewall as a percentage of the width.

Edited by PositronicRay on Friday 14th June 15:40

[CHEF_GOLF]

Aaahhh makes sense now. Thank you for clearing it up.