One of the back wheels on my car has a different rim size than the others. Specifically that odd wheel is 195/65R15 whereas the other three tires are 205/55R16. I calculated the total diameter of the wheel with these tire/rim configurations and the total difference (theoretically) is about a tenth of an inch (0.1")

Is this okay to drive on permanently?

No shop has ever noticed or mentioned it to me, in fact it even took myself several months to notice they were in fact different.

For reference, I am driving a 2006 VW Jetta 2.5 (manual transmission). The three wheels (two front, one back) are Blizzak WS60 205/55R16 91R. The odd wheel is a Michelin Energy MXV4 S8 195/65R15 91H.

  • Not knowing what the make/model/year of your vehicle is makes it really hard to say. Also, the exact make/model of the tire would be of great assistance as well (to determine exact rotations per mile). You don't want to put the odd ball tire on the drive axle, as this will cause issues with the differential. You wouldn't want it up front because it will affect how the car drives. Tire rotations are out. Give us the rest of the info and we can help you here. Jun 25 '15 at 1:03
  • Edited it in. The odd Michelin wheel I was able to find a spec for, says 832 rpm. The Blizzak tires I couldn't find an exact spec for, but interpolation nearby specs I approximate it at 837 rpm. The mismatched pair is on the rear axle as my car is a front-wheel drive.
    – Andrew Lee
    Jun 25 '15 at 2:21
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    If it's on the non-driving axle, don't even sweat it. It's well within the tolerance level of the drive differential at any rate, but non-driving wheels are pretty liberal when it comes to differences in diameter. As long as one of the wheels isn't comically over-or undersized (e.g. an SUV wheel on one side and a Korean econobox wheel on the other) you won't have any problems. Jun 25 '15 at 10:01
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    All that flies out the window for the drive axle, and especially so if you've got a 4wd. One of the reasons my next car is going to be a FWD again. I like my Subaru and all, but even a slightly deflated wheel can do expensive damage to the center differential (aka the "viscous coupling"). I don't need that drama in my life. Jun 25 '15 at 10:06
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    If this vehicle was in the UK, this would be a valid cause to fail an MOT (annual roadworthyness inspection). It's specifically declared in the regulations that tyre sizes must not be mixed across a single axle. Jun 25 '15 at 11:01

There are several issues to consider here. In the first place, there is the question of transmission from the engine/gearbox to the wheels. Most vehicles are designed so that driven wheels run at the same speed, so need to be precisely the same diameter. Otherwise, differentials need to work continuously to compensate for the difference in wheel rotation speeds Exceptions are some models of agricultural vehicles, with different size wheels front/rear. But the drivetrains on these vehicles have been designed to take this difference into account.

In your case, the different size wheels are on the rear axle of a front wheel-driven car, so this is not a problem. Things would be different on a rear wheel drive, or permanent 4x4 vehicle. I have experienced transmission noises on a Honda 4x4 Beagle in similar circumstances.

The second aspect to take into account is handling. Wheels of different widths handle differently in curves, even if their overall diameter is more or less the same. Even different makes of wheel rim can have their influence, e.g. with rims of differing weights. So your car will handle in a slightly different way when going around bends to the right or to the left. If going really hard, the rear will start to slip at different speeds. However, if you drive cautiously and at sensible speeds, this should not be a problem.

Note also that your tyres are in fact a mix of winter tyres (the Blizzak) and summer tyres (the Michelin). They have completely different surface patterns and compositions. You can expect very different handling characteristics.

As a side note, what has been said so far is applicable to spare wheels with different sizes from regular car wheels. These are always marked with specific speed limits: "Do not drive in excess of 80 km/h or 50 mph", "Exercise caution", etc.

Finally, depending on the specific legislation of the country (or state) in which the vehicle is registered, you could very well be foul of the law. For instance, in my country it is clearly specified that all wheels on the same axis must be of the same type and characteristics. Even using tyres of different brands is frowned upon, since wheel sizes may present some variation between tyre manufacturers or even wheel models.

If you are concerned with this situation (I would - specifically with the difference in rim sizes), a simple solution would be to purchase one complete wheel of the same type and brand as the three identical wheels already mounted on the vehicle. You then have four equal wheels to drive on. The different wheel can be re-purposed as a spare, to be used only in the case of a flat as a means to get you to the garage.

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