2018 B9 A4 Allroad tire size question

[chakatrain]

Hi Folks,

It’s my first post here; I’m glad I found y’all.

I picked up a 2018 Audi A4 Allroad this week. It’s on OEM Continental ProContact TX 245/45/18 tires on the OEM 18x7.5” rims.

I have a set of barely used 225/45/18 Michelin Pilot Sport All Season 4s I pulled off the car I traded in for the B9 A4 allroad. From what I read about matching tires to rim sizes, seems like a good rule of thumb is to match the tire tread width to be the same or maybe just a little smaller than the rim width. The 225/45/18 Michelins have a tread with of 7.2”, which would seem to sit nicely on the OEM 7.5” wide wheels. However, the OEM wheels have an 8” tread width, 1/2” inch wider than the rim width. Audi had a reason for spec’ing the car this way; I would think this might contribute to a more floaty ride feel.

I’m in California and will use the car for road trips and camping trips, with only light dirt roads to get to the good camping spots. I’ll likely see snow at Lake Tahoe area only 2-3 times a year on weekend trips.

What do you Audi owners advise? Would it be ok (or even smarter) to put on the 225/45/18 Michelins or should I buy a new set at the OEM size (245/45/18)? I know the diameter of the 225/45/18 will be smaller, which will affect the speedometer, wheel gap and ground clearance. I’m just curious if the 225 tire’s tread with of 7.2” would make for a more planted ride or if it would be too compromised for what the car was originally designed to be?

Thanks!

[gk1]

There is more to what tire they chose than just width and comfort.
Load rating being the most important.
The OE Continental ProContact TX 245/45/18 is XL load rated at 100 = 1764 lbs
The Michelin Pilot Sport All Season 4 225/45/18 is only XL load rated to 95 = 1521 lbs.

Will that make a difference, perhaps not.
However, I think general rule of thumb for most tire places is to NOT install a lower load rated tire than what came stock as they could be liable if there is an issue.

We can try an actual scenario where it might work.
Let's say you use the Audi "normal" load pressure with the 245/45/18's of 32/32 psi...that gives you a load capacity of 1418 lbs.
You have to inflate the 225/45/18 to 39 psi to get an equivalent load rating which may be an uncomfortable ride.
You could lower it to make it more comfortable, but sacrifices load capacity and at what point does that became an issue?
Is it say 27 psi when the load rating equals 1/4 of the weight of the vehicle with two passengers? I'm not sure. I bet there is a large safety margin built in, but not really sure if you are looking to test that out.

And for max load rating Audi states 35 psi on OE tires which means 42 psi on the 225 and it is at its technical load limit...so you can see why Audi adds in that extra margin of error with the larger size and higher load rated tires.

To your other points:

Yes, 225/45/18 is almost 3% smaller than the OE size so when your speedo read 60 you will actually be doing 58, plus you will have more wheel gap which may not be an issue because allroad.
However after all that...225 is fine on a 7.5" wide rim it is actually the manufacturers measuring rim width. The 245 is measure on an 8" wide rim and is fine in the range down to 7.5".

IMO I'd get new 245's since the combination of the correct size and load rated tire with Michelin's reportedly better design and rubber will be the best of all worlds.
However, if you want less float and more solid ride the best way I can think of to achieve that is to go for larger wheels.
A 245/40/19 (OE spec) has 12mm less sidewall and should offer a slightly more sporty feel or step it up to a 20" with 245/35/20 (24mm less sidewall).
This will also sacrifice ride quality and load rating drops too, so you will have to again increase pressure.
Hope this helps.

[chakatrain]

That is very helpful. I hadn’t considered load rating at all (am still learning).

I do see that the general consensus online about not putting on tires with a lower load rating than the OEM tires. Thanks for taking the time to help put all that out there, too!