Replacing SUV tire(s)

Well, they got what they wanted... your $. Did you get what you wanted?

If the 1st tire was damaged enough to replace, then yes replace it. The tread depth on most tires is around 5/16"... If the tire you damaged was a 265/65R16 ( just for example) was half worn and you put a new one on, the difference in tread height would be maybe 3/16".... let's do some math 265 / 25 = 10.6" * .65 = 6.89" * 2 = 13.78" + 16 = 29.78" tire diameter .. 29.78 * pi (3.1415) = 93.55" circumference..... 93.55 /

12 = 7.79' .... 5280 / 7.79 = 677 revolutions per mile for your new tire. 29.78 less 3/16"(.1875) = 29.59" diameter * pi = 92.92" /12" = 7.74'... 5280 / 7.74' = 682 rev per mile...... So you old tires would revolve 5 turns more per mile than the new one. I don't think that would be too bad considering what cornering, turns and scrubbing would to to negate any ill effects on a hefty a** third member of a 4WD. So two tires would have been my choice. The other two wouldn't have made an iota of difference. An AWD car is managed by the transfer case anyway and a 4WD shouldn't always be in 4WD, unless it's off road or in snow, etc....

Congratulations, more than likely you've just been ripped off for three tires. Though to be perfectly honest you didn't state how worn the old tires were, and if they were down to the wear-bars the tire store had a valid reason to sell you at least two.

You do not mention exactly what make and model your car is, but the ONLY time the tires must be the same model and size and fairly close together in tread wear (outside circumference) is when the car has a Full-Time All-Wheel-Drive system without a center differential, and those are fairly rare.

Is yours is a standard part-time four-wheel drive system where you have to hit a button or pull a transfer-case lever and manually engage the front hubs to go into 4WD? If so the tires do need to be the same make and model and basic tire size, but the exact tire diameters from wear are not overly crucial. As long as they aren't worn below the safe minimum tread or otherwise coming apart, they're fine.

The tires will always spin at slightly different speeds from cornering, but you are in the dirt, mud, snow, etc. and the wheel slip on the reduced traction surfaces will relieve any possible drivetrain wrap-up stresses.

Then again, Sears has been busted over and over for selling unneeded parts and services - from replacing "worn brakes" that were almost new right down to those fraudulent practices from the 1930's that still work in 2006 (if nobody spots them), like squirting a bit of oil on the shock absorbers to show you that 'the shocks are leaking and must be replaced'.

The California DMV Bureau of Automotive Repair almost yanked Sears' licenses several times. Along with several other big chain stores.

Here's the "Money Paragraph" -

Seems that every Mechanic and Customer Service Rep at these national and regional chain stores has a quota of sales to meet. Corporate wants each and every employee to push every service they see that is applicable to get add-on sales out of every single customer that walks in the door, so your report is not surprising.

Jiffy Lube gets you in the door for a '$19.99 Oil Change Special', but their corporate policy is to sell at least $60 to every customer - and to do that "every 3,000 Miles", of course.

And if they have to stretch the rules and cite nonexistent "safety rules" and "federal laws" to make that "oversell" sale, they'll do it. If they quote a rule or law to get you to buy, you have every right to have them show it to you in writing - put up or shut up.

Some of it is to avoid liability suits - their lawyers not wanting to let you out the door on marginal tires where they can get sued after the accident. But most of it is pure "revenue enhancement".

They operate under the age-old principle that there's a sucker born every minute. You have to remain eternally vigilant to ensure you aren't one of those suckers.

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Actually, my 2001 Subaru did have a center diff, and they had some clearly stated specs for tire replacement and allowable circumference differences.

I don't think it's all that rare for AWD cars.

The tread depth on most new tires is actually closer to 5/16" for passenger car tires and 11/32 or 4/8" for truck tires.

AWD vehicles generally have a center differential; 4WD vehicles have a transfer case.

Are you kidding me?

a 1/8" difference in tread size is NOT going to throw the differential off that much! I could see if you had, say, 225/60-16 on one axle and

235/60-16 on antoher axle, but the difference in tread (from the same make and model tire) is not going to make that much difference!

I have 205/75-15s on the front of my Grand Voyager, and 215/70-15's on the rear. The system is full time AWD with a fluid trtansfer in the middle of the van. Close enough for government work!

And, if you have a Transfer Case, chances are you're in RWD except when the going gets slippery, and when it DOES get slippery, it gives the tires enough leeway to not make a difference over tread depth!

The car is a 1999 Mercedes ML320 SUV. The two rear tires (different axle than the busted tire) were in relatively good shape, not near the wear bars. The other front tire I could make a case for replacing, and I did suggest that option to the sales rep and again he said he still wouldn't mount them for me; sell them to me yes, mount them on the car - no.

Walter

----- Original Message ----- From: "Walter Cohen" Newsgroups: alt.autos.toyota,alt.autos Sent: Sunday, October 01, 2006 4:13 PM Subject: Replacing SUV tire(s)

What was your vehicle? What size was your doughnut tire? If driving around with different size tires was likely to damage the 4WD system, then the vehicle manufacturer would have included a spare with the same OD.

There is an interesting thing about radial tires that comes in to play. The OD of the tire has only a secondary affect on the rolling radius (the dimension that affect the number of revolutions per mile). The tread sections act more like a tank tread than the outside diameter of a solid wheel. The actual rolling radius is mostly dependent on the radius of the steel belt instead of the outside diameter of the tread. For instance, a Michelin CrossTerrain P235R16 Tire has an outside diameter of 29". If this was a hard wheel with a 29" diameter, it would take 695.4 revolutions of the wheel to go one mile. According to the Michelin specs, the actual number of revolutions per mile is 720 (at 45 mph - speed is a factor because it changes the tire diameter slightly). 720 revolutions per mile gives you an effective diameter of 28". This is close to the diameter of the steel belt inside the rubber. This diameter of the steel belt is not effected by wearing the tread down. There is some slight change in rolling diameter as the tread is worn away, but not to the extent the outside diameter change would indicate. In the case of the Michelin P235R16 tires, the tread depth, was 10/32. Even if the effective rolling diameter was decreased from 29" to

28.4" (new to bald), the theoretical number of revolutions per mile would have only changed from 695.4 to 710.1. Any system that can't handle 15 revolutions in a mile is already defective. Going around corners is a more significant difference than that. And in actual fact the difference in effective rolling diameter between the new tire and the old tire (assuming the same brand, size, and type) was probably more like 5 revolutions per mile and probably less.

I think it was justifiable to replace two tires (the bad one and the other one on the same axle). Replacing four was totally unnecessary. And didn't warning bells go off when this tire store forced you to buy four tires since

18 months earlier you had already just replaced two of the four?

Ed