Tire diameter

[googleman]

Hello All,

I was switching my summer 18 inches with my winter stock 17" tire-wheel combos. After doing 3 wheels, I realized the last one has a stripped lug nut! Would there be any problem for the AWD if I drive about a mile to a shop with 3 17"s and 1 rear 18" installed on the car? If the online tire diameter calculators are right:

18" diameter: 27.74

17" diameter: 27.63

I know AWD's should be run with matching tires and wheels, but just curious if anything would be damaged for such a short distance with very close tire diameters.

Also, a related question. When I use my summer 18"s, will the stock spare tire work without a problem if needed? It came with my stock 17"s.

[CDNicecube]

I would not if I can drive another car because I am paranoid with these things. But assuming this is not possible, and others might disagree, I believed it Is most likely be allright because I am sure that many drivers drive awd cars with one or more tires deflated by more than the difference you are noting.

On this thought, perhaps you can inflate the 18" by a few extra pound. But I would drive very very slowly under 20km/h either way.

Really our cars are not true 4WD.

[googleman]

I would not if I can drive another car because I am paranoid with these things. But assuming this is not possible, and others might disagree, I believed it Is most likely be allright because I am sure that many drivers drive awd cars with one or more tires deflated by more than the difference you are noting.

On this thought, perhaps you can inflate the 18" by a few extra pound. But I would drive very very slowly under 20km/h either way.

Really our cars are not true 4WD.

The spare tire -- according to the manual -- is T165/80/R17. That makes a diameter of 27.39, smaller than either the 17 or 18 diameter that I listed above. The weight difference between the 17 and 18 wheels should also be smaller than the difference between either and the stock. So I feel like I should be fine to drive it for a mile to a shop to have the lug nut removed.

Great suggestion on inflating the 18 a bit more, will do that tomorrow morning just to be sure.

I read somewhere that our ATESSA AWD system tolerance for tire diameter difference is 4mm. 0.11inch is within that limit, but even everyday psi variations on matching tires could cause that difference. Subaru has a bit more tolerance apparently, but still, maybe a 4WD is the way to go.

[EXceptional]

Me thinks you'll be OK too, if you're on dry pavement. If your traction is 100%, then there's no reason for all the stuff in the next paragraph to be going on. I do believe the car always starts out in AWD mode and then becomes a 2WD later. If your traction is 100% and your mis-matched wheel in on the front, you'll be OK.

(Quoted from another thread...)

"To answer your question, the EX's AWD system does have the ability to transfer power to a particular wheel using the anti-lock braking system (I know it can do this with the rear wheels, but I'm not sure about the front wheels since it could have negative impact on steering the vehicle...I will have to read-up on this). So, when slippage occurs with one rear wheel, the ABS kicks-in to reduce wheel spin, thus "transferring" torque to the opposing wheel. It's kind of a modern day "limited slip differential" if you're familiar with those. As with any traction management system (e.g., AWD, limited slip, etc), if the wheel receiving the torque doesn't have much traction either, your vehicle will likely slip and slide a little as the system tries to shift torque around to the wheel(s) with some grip, but that's the nature of the beast--and even more reason why appropriate tires for your region and driving conditions are so important."

AWD Video = https://www.youtube.com/watch?v=Eanc_PbMDDk

[NJGuy]

Given the really small difference in wheel diameter and the 1 mile distance to the shop, I wouldn't worry at all. It's not like you're doing highway driving on mismatched wheels for weeks at a time.

[googleman]

Thanks a lot everyone. Got it fixed at a nearby shop today with no issues on the way. Measured the diameters before driving, and probably because the compound on the snow tires is softer, the actual diameter difference between 17 and 18 was even smaller.