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dotNetBB Forums > 1010TIRES FORUMS > Tires Technical > What is the brake failure risk for substituting tires smaller than recommended 3% diff in diameter

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V8TR6
Registered Member

Date Joined Oct 2009
Total Posts : 2

Posted 10/26/2009 2:26 PM (GMT -7)

I have a set of (4) 225/60/16 winter tires mounted on winter rims from my wife's last car. The tires only have about 12000km and are in great shape. I have now purchased a 2005 Jeep Liberty for my wife that has the upgraded 235//65/17 tires, other size for the Libery is 225/75/16.

When I used the Tire Size Calculator to see if I can use the winter tires on the Liberty, I find that the diameter difference is -8.28% which gives a speedometer error of +9%. I know this exceeds by a large margin the recommended 3% maximum. In fact the caution note from the calculator warns of risk of brake failure.

I do not understand why running the winter tires 9% faster at normal highway speed would be any different than if the speed limit was exceeded by 9% ?? Why would the brakes be at risk. The tires do fit the Liberty - I checked last night.

I would defintely like to re-use this set of slightly worn winter tires on the Jeep but not at the expense of endangering my wife with a brake failure.

I would appreciate an explanation about the brake failure risk.

Thanks for any help.

CapriRacer
Forum Moderator

Date Joined Mar 2004
Total Posts : 462

Posted 10/27/2009 4:05 AM (GMT -7)

I'm not going to address the ABS issue - because I just don't know enough to explain why some ABS systems act up in those circumstances. But I am going to address the load carrying capacity issue - which, typically, isn't mentioned.

Going down in load carrying capacity increases the risk of a load related tire failure. Of course, the further away from the original load carrying capacity, the greater the risk. The problem is that tire failures sometimes result in tragic accidents. In a Jeep, this risk is accentuated.

I think the risk involved with the load carrying capacity issue is high enough to recommend against using the tires - all by itself.

CapriRacer

Post Edited (CapriRacer) : 10/27/2009 11:10:48 AM GMT

V8TR6
Registered Member

Date Joined Oct 2009
Total Posts : 2

Posted 11/1/2009 9:31 PM (GMT -7)

Thanks for the prompt reply.

The tire load rating in this case was not going to be an issue as the Jeep was not going to be loaded anywhere near the load rating of the smaller tires in winter driving.

Have talked to several tire people and no one can give a satisfactory answer why there may be any problems with the brakes exceeding the 3% difference rule. One thought that the ABS might be affected but he did not know how.

As per your expressed concern, the primary reason for the 3% rule seems to be to ensure that tires with insufficient load ratings are not used.

The affect on the speedometer reading would be another reason but this would only be an inconvenience. With a GPS unit in the car, could easily determine correct speed.

However, in the interests of ensuring safety, I have sold the old set of tires and rims and have bought a new set for the Jeep in the recommended size.

Thanks for providing valuable tire advice on this forum.

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