spare tire and size difference

[rh71]

I run on aftermarket 20" rims but stock was 19" and so I assume the full-size spare is also 19". I only quickly measured the diameter of both tires and they were 28". It's pouring out or I would find the spare's measurements on the sidewall but it was upside down and tight in there. Is there any reason why I can't run on the spare in a pinch? It's an AWD car and I read that may present a problem for the tranny/diff with the size differences - which may be very small using my unscientific measurement.

Just trying to figure out if there's even a purpose to carrying the spare around if it's a mismatch.

How does all this work with smaller size donut tires in the past? Wouldn't that affect the tranny also?

 

[Mark R]

It's preferable if all the wheel/tires are the same size.

However, a small mismatch isn't terrible for short-term, low speed use. It'll ruin the handling, as the suspension won't be level, and it will put a lot of load on the differential due to the continual different rotation speeds.

But if you keep below 50 mph and only drive a few hundred miles, you'll be fine - and that's a lot better than being stranded without a spare.

Donuts are worse, because they are seriously undersized for the load they need to support - they are often made of a super soft compound, with super stiff sidewalls. Typically, they only have a rated max speed of 40-50 mph and a maximum recommended lifetime of 50 miles, because of the extreme stress.

 

[exdeath]

Depends on your drive train and differential setup.

The concern with tire size mismatch has to do with causing a limited slip differential to remain in a constant state of slip (eg: as if you were turning) and it will overheat and fail.

It's recommended that if the axle you are replacing the tire on is LSD, you need to swap a good wheel of the same size off a non LSD axle first and then put the spare on the non LSD axle. If the vehicle is AWD well.. you are screwed.

In an emergency you will be ok taking it slow and driving only a short distance to park it, but the idea is not burning up a limited slip differential or transfer case. They are meant to slip only temporarily or periodically while turning, not while flying down the highway for two hours.

If you can run that wheel/axle open you can put whatever size wheel you want on it. (in before someone shouts "but wheel bearing will wear faster!!1!")

 

[imagoon]

Some AWD systems have a disable switch / fuse / magic trick for this reason. Subaru has a fuse that you put in for example. They also specify which axle a spare is "safe" on. Mazda AWD says rear only and rear diff disable on some cars as another example.

 

[LTC8K6]

I only quickly measured the diameter of both tires and they were 28"

If the spare is the same outside diameter as the tires you are using, then it's ok to use it in an emergency.

Some AWD systems are sensitive to the point that tire wear can make a difference, such as putting a new spare with three 30k mile tires. Read your owner's manual.

 

[kitatech]

This site will calculate the actual difference between the 2 tires:

http://www.rimsntires.com/specspro.jsp