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Tire Talk: Lowering load index on sidewall is dangerous

The Toronto Star's John Mahler answers your questions on Tires for Wheels.ca. This Week: Lowering load index on sidewall is dangerous

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Tire Talk

I have a Mercedes GL500 SUV with a tire size of P295/40R21. There is a bubble in the left rear tire about the size of an open hand. I?ve driven about 50,000 km on the car and I think it?s time to replace all four tires. I found a Michelin (size P295/35R21) with a good price discount. Can I use it on the car?

No, under no circumstances can you use this tire. It is not safe. As sidewall height goes down, load carrying capacity goes down as well.

Your OE tire has a load index of 111 (2,403 pounds per tire). The Michelin you are looking at is only rated for 107 (2,149 pounds). The GL500 is a heavy vehicle that works its tires hard. Dropping this kind of load index can cause your tires to overheat and possibly fail.

The load index always totals more than the weight of the car. That is because the load shifts around quite dramatically. When cornering, much of the car?s weight is carried by the two outside tires. Under hard braking, weight shifts forward and the front two tires can carry the majority of the weight.

Get the correct size and load index. Sizes can sometimes be changed but the load index can never be lowered.

Tire Talk: Lowering load index on sidewall is dangerous

 

I am looking at replacing the Continental ContiProContact tires on my Chrysler 300. I?ve been happy with the tires but am not sure if I should stay with them or go to something equivalent from Michelin. Also, I?m wondering if I should move from an H-rated tire to a V-rated one.

I?d be inclined to stay with the H-rated tires as OE spec. You might want to consider changing brands for a better ride.

Unless you are really pushing the car through corners, the performance difference between H and V is small, but the ride difference can be felt. Another factor to consider is, the higher the speed rating the faster the wear.

Even in the same speed rating between different companies, the ride quality can vary a lot. Michelin, for instance, puts ride quality near the top of its list. So you may enjoy the ride of the Primacy MXM4 more that the Conti.

But everything is a trade-off. Nicer ride means less-sharp response, although that may only be noticeable in extreme conditions.

Send tire questions to: thetireguy_1@hotmail.com. Mail volume prevents personal replies.

More Tire Talk:

Tire comparison: Continental TrueContact vs. Bridgestone Ecopia

The Toronto Star for Wheels.ca

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