Wheel rim and tyre size compatibility

[Pappa583]

My q7 was a 5 seater from factory and Just noticed my spare tyre size is different. The new tyre, squashed down and deflated, never been used but realised the tyres fitted to the car are 185,40,21s but the spare is a 185,65,20.....

Take it the wheels were upgraded by previous owner from 20s to 21s.

Understand even having one new tyre fitted to one wheel and leaving a worn tyre on the same axle could cause issues with the quattro drive system? How do the sizes compare with overall diameter as don't want to use the spare if I ever needed to and end up damaging the drivetrain...

[Magnet]

Hello Paul,

Now you are testing the maths of an old timer here! - but my calculations point me towards your 20 inch spare (when it’s inflated, and it’s of no use deflated as it is) will be 67 mm greater in diameter than your in-use 21 inch low profile tyres and wheels. That’s a lot , and too much in my book, but please check by inflating the spare and physically checking one against the other.
Have you had a look at the handbook to check the standard wheel/tyre combination, and the suitable alternative sizes that could have been chosen?  
Kind regards,

Gareth. 

[Pappa583]

1 hour ago, Magnet said:

Hello Paul,

Now you are testing the maths of an old timer here! - but my calculations point me towards your 20 inch spare (when it’s inflated, and it’s of no use deflated as it is) will be 67 mm greater in diameter than your in-use 21 inch low profile tyres and wheels. That’s a lot , and too much in my book, but please check by inflating the spare and physically checking one against the other.
Have you had a look at the handbook to check the standard wheel/tyre combination, and the suitable alternative sizes that could have been chosen?  
Kind regards,

Gareth. 

Hi Gareth,

Your calculations better than mine....I did think there would be a difference and being that big a difference best not using it as it would damage the drive train...

think the issue is the lad who had the car before me put 21" wheels on but the spare is from original. Reason it's deflated is its almost like a vacuum packed spare tyre so it fits inside the boot floor. The tyre is on the rim but the side wall is sort of folded so the tyre is smaller to fit in. If I inflate it wil never go back in 🤣

Might aswell remove it from the boot and store the spare in the garage for now, gives me some storeage aswell

[Magnet]

Thanks Paul,

The question that remains is whether 21 inch wheels were listed as an option, and if so what size tyre would have been the correct fitment. 
It could be me, but I remain surprised that such a large vehicle would be running on such narrow (185) tyres. Even our humble A3 was fitted with 205 wide tyres. The handbook/ Google search should come up with the answers.

Kind regards,

Gareth. 

[Pappa583]

11 minutes ago, Magnet said:

Thanks Paul,

The question that remains is whether 21 inch wheels were listed as an option, and if so what size tyre would have been the correct fitment. 
It could be me, but I remain surprised that such a large vehicle would be running on such narrow (185) tyres. Even our humble A3 was fitted with 205 wide tyres. The handbook/ Google search should come up with the answers.

Kind regards,

Gareth. 

What an idiot I am.... sorry they are 285 not 185. From the audi app, spec for wheels on car from factory  were 285.45.20.

Doesn't make much sense as even that wouldn't seem to be a match with the spare. Suppose either way whether it was OK with original wheels its not right with having 21s fitted now.... 

Either run flats when the tyres are due a change or some cans or foam.... 

[Magnet]

That’s better! A 40 profile tyre on the same width 21 inch wheel would sound about right, but you need to confirm via. an alternative diameter wheel suitability search. 
The larger the wheel, the lower the profile, the harsher the ride. 
I would think seriously about choosing XL tyres on 40 profile - you’ll end up by believing you are driving on solid rubber. Having said that, I wouldn’t be at all surprised that the original spec. for such a vehicle would have been XL anyway, so it would be worth checking that, and whether your current tyres are XL. 
A bit of homework to do! 
Kind regards,

 Gareth.

[Pappa583]

Didn't realise they would be so much different

See the link, useful for anyone thinking of changing wheels and tyres from standard, shows speedo differences also.

https://tiresize.com/comparison/