Mixing Tire Sizes (Winter Rubber)

buckyNovember 12, 2009

My FWD Chevy currently has 225x50R-17 Blizzak winter tires on the front for winter driving. I've been running these tires on the front with a pair of the original all seasons on the rear for a few years and they've worked fine. A new pair (2) of exactly the same Blizzak winter tire make and model have been offerred to me at no cost but they are 235x45R-17s so they're a little wider and a bit lower profile than the front set. Replacing the two rear all seasons with these Blizzaks will give me much improved rear traction, but I'm not sure how it would affect the vehicles handling with different sized tires front and back. I realize the ideal situation would be to have all four tires the same size but these tires are real costly so if I can use the new ones offerred to me for free I'd like to... but not if it seriously compromises handling and therefore safety. Any thoughts on this fellas?

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The 235 45R-17 tires are slightly less in diameter than the 225 50R-17 tires. If used on the front, your speedometer readout will be affected by 2.1% It will indicate a greater speed than actual. Calculations below:

225 50R -17
Tire diameter: 17 + (2x225x0.5/25.4) = 25.86 inches

235 45R -17
Tire diameter: 17 + (2x235x0.45/25.4) = 25.33 inches

The speedometer will read fast by 25.86/25.33 or 1.0209
or 102.09%.

At indicated speed of 60 mph, the actual speed would be:

60 x 25.33 / 25.86 = 58.77 mph.

But, since these will be on the rear, therefore handling and wheel well clearance are the main issues. Wheel well clearance gains importance when snow packs into the wheel well. The 235 45R tires are wider by 10mm or about 0.4 inches; That would be 0.2 inches per side. If you have ample clearance, this litle difference can be accomodated.

The effect on hamdling is unknown to me. There should be little change if the sideways stiffness of the two tires are the same. However, if the replacement tires are more flexible in the side-to-side direction, it may cause the vehicle to 'hunt' while at speed, e.g., wanders from side to side and require constant steering correction to stay in your lane.

A few reports that I have read states that the ride is stiffer with the lower profile tires. I have no experience therein.

    Bookmark   November 12, 2009 at 6:42PM
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>>The speedometer will read fast by 25.86/25.33 or 1.0209
or 102.09%.Uh! 2.09% maybe? :-)

    Bookmark   November 12, 2009 at 10:28PM
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I have heard too that the lower the profile of the tyre the harsher the ride. That is down to there not being as much sidewall to absorb the bumps on a lower profile tyre as there would be on regular tyres. This is further hampered by the fact that cars that come with low profile tyres as standard are generally more sporty models that tend to have suspension set up for the track rather than the public highway. Horses for courses.

Anyway, to address the initial subject of the posting, I am told there is nothing wrong with having one size on the front and a different size on the back. That is legal (well it is here anyway). It is the mixing of different sizes on the same axle that is a no-no and will get your collar felt by the cops if you manage to stay out of the ditches and bushes long enough to get stopped in a vehicle of that set-up. And when it comes to accuracy of the speedometer which will be affected if you change from standard fit the tyre size on the driven wheels, your speedo can over read as much as it likes but it mustn't record a speed lower than you are actually doing.

    Bookmark   November 13, 2009 at 7:14PM
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The other concern is what size do you carry for a spare? Most folks carry only one spare. I suppose you should carry a spare that matches the driving wheels.

    Bookmark   November 14, 2009 at 2:43AM
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