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Discussion Starter #1
I got the car last year popped one of the front tires and only had the money to replace the front 2. Today I wen to get some new back tires because they are bald and im going on a 400 mile road trip to Ohio in a few days. The tire place told me I neede to replace all 4 tires on any all wheel drive car. I have a 99 GSX. I thought they were bull shitting me so I went to another place where they put 2 new tires on the rear of my car for me. So my question is will this really ruin my driveline or whatever that guy was telling me about?
 

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Yes, you need to replace all 4. You will damage your drive line if you dont.

Car Talk: Shortcut on tires will end up being very costly

Dear Tom and Ray: I recently had four half-worn snow tires on my '04 Honda CR-V with all-wheel drive. Living in upstate New York, I was driving in snow on back roads much of the time (as a rural letter carrier for the U.S. Postal Service). I had the bright idea to buy two new snow tires this year for the front wheels and use the worn tires for the rear, and then next season reinstall the newer tires on the rear and buy new tires for the front. That would eliminate having to buy four new snow tires at one time. However, the local Honda service manager advised against my plan, saying that the newer tires would have a slightly larger radius than the older tires on the rear (even though the actual tire sizes are identical), and this could cause problems with the all-wheel-drive system. Is he correct? -- Bruce

Ray: Sadly for you, Bruce, we have to say yes. However, think of all the tire dealers who are clicking their heels while reading this!

Tom: The anecdotal evidence suggests that all-wheel-drive systems can be damaged by using different-size tires on different wheels. Even if the tires started out as exactly the same size, if one pair is worn down, that difference in circumference can cause problems.

Ray: Here's what happens. On all-wheel-drive cars -- cars that are permanently in four-wheel drive or can be left in four-wheel drive all the time -- there's a part called a center differential. This is a clutch assembly or viscous coupling that's so complicated, we couldn't possibly explain how it works without using our hands. And maybe the hands of at least two or three innocent bystanders.

Tom: When the car is going straight down the road, the center differential doesn't have to do anything. It just sits there. But when the car turns, the front and rear axles need to turn at different speeds to keep the wheels from binding up and the car from flipping over. The job of the center differential is to allow the axles to turn at different speeds.

Ray: The problem is that if the front and rear tires are different sizes, then the front and rear axles are always turning at different speeds. Even when the car is going straight ahead. That means the center differential is always working -- and that wears it out.

Tom: And the center differential costs big money.

Ray: Maybe someday they'll make center differentials more robust, so they can take the constant use that different size tires require. However, that's not how they're made at the moment.

Tom: So it's better to be safe than broke, Bruce. Take the service manager's advice and replace all four tires at once. We would hate to see the Postal Service have to change its motto to "Neither snow nor rain nor heat nor gloom of night, nor a $900 center-differential replacement ... stays these couriers from the swift completion of their appointed rounds.
because Suburu's AWD is different from 4WD (4x4)

Because Suburu's AWD system is "on" all the time.. engine power goes from front transaxle directly to rear differential. This system Depends on all 4 tires being the SAME size. If any of the tires is the Wrong size.. it can hurt your transmission, axles, differentials, or all three! The only "give" in such a full-time All-Wheel-Drive system is the clutch, and limited slip systems in the rear differential and front transaxle.. they can take "Some" difference in speed, ie when turning, or 1 or 2 wheels spin in a puddle.. but if 1 or 2 tires is the wrong size then there will be an Incredible excess of load & heat on the clutch and slip systems.

Although the same thing *can* happen with 4x4 trucks.. the difference is with 4WD trucks, the front hubs can be disengaged and the vehicle will run RWD-only. This type of system can unload the burden from the front differential until it is activated for off-road use.. and in that case the dirt/mud of offroad use will act as a "give" point for any uneven tires sizes or rotational speed.

However, full-time AWD systems do not have that luxury. And thus the same wheel/tire sizes at ALL 4 corners is CRUCIAL to the longevity of the vehicle's entire drivetrain! :)
he ability of four-wheel drive and all-wheel drive vehicles to divide the engine's horsepower between its four tires is especially useful on loose or slippery surfaces such as sand and dirt, as well as on wet, icy or snow-covered roads. However it's important to remember that in order to transfer this extra power, the four-wheel drive and all-wheel drive vehicle's driveline mechanically connects the tires so they work in unison.

Four-wheel drive and all-wheel drive vehicles are equipped with additional differentials and/or viscous couplings that are designed to allow momentary differences in wheel speeds when the vehicle turns a corner or temporarily spins a tire. However, if the differentials or viscous couplings are forced to operate 100% of the time because of mismatched tires, they will experience excessive heat and unwarranted wear until they fail.

This necessitates that four-wheel drive and all-wheel drive vehicles use tires that are very closely matched. This is because different diameter tires roll a different number of times each mile as a result of the variations in their circumferences. Tire diameter variations can be caused by accidentally using different sized tires, tires with different tread designs, tires made by different manufacturers, different inflation pressures or even tires worn to different tread depths.

As an example of different tire diameters resulting from tires worn to different tread depths, we'll compare two 225/45R17-sized tires, a new tire with its original tread depth of 10/32-inch and a second tire worn to 8/32-inch of remaining tread depth. The new 225/45R17-sized tire has a calculated diameter of 24.97", a circumference of 78.44" and will roll 835 times each mile. The same tire worn to 8/32-inch of remaining tread depth is calculated to be 1/8" shorter with a diameter of 24.84", have a circumference of 78.04" and will roll 839 times per mile. While the difference of 1/8" in overall diameter doesn't seem excessive, the resulting 4 revolutions per mile difference can place a continuous strain on the tires and vehicle's driveline. Obviously, the greater the difference in the tires' circumferences, the greater the resulting strain.

This makes maintaining the vehicle manufacturer's recommended tire inflation pressures and using "matched" tires on all wheel positions necessary procedures to reduce strain on the vehicle's driveline. Using "matched" tires means all four tires are the same brand, design and tread depth. Mixing tire brands, tread designs and tread depths may cause components in the vehicle's driveline to fail.

Mismatched tires or using improper inflation pressures for all-wheel drive and four-wheel drive vehicles can also result in immediate drivability problems. Some Control Trac equipped vehicles in 4Auto mode may exhibit a shutter on acceleration and/or a noise from the front driveline and transfer case while driving. Some all-wheel drive and four-wheel drive vehicles may exhibit axle windup or binding while driving. Some four-wheel drive vehicles (manual or electronic shift) with a two-wheel drive mode may refuse to shift "on the fly" into 4x4 Auto or 4x4 High at highway speeds.

Rotating Tires

Because the front and rear tires of all-wheel drive and four-wheel drive vehicles perform different duties while accelerating, braking and cornering, tire rotation is important to even out, and maintain equivalent treadwear of all of the vehicle's tires to minimize potential driveline stresses.

If the vehicle is equipped with a matching road wheel and full-sized spare tire, they should be integrated into the vehicle's rotation pattern at the first tire rotation. This will allow all five of the vehicle's tires to share in the workload and wear at similar rates. In the event that a single tire is damaged and has to be removed from service, this will allow the tread depth equivalent spare tire to be used with the remaining three tires.

If the spare is not integrated into the vehicle's tire rotation pattern, it will not match the tread depth of the four worn tires on the ground when called into service. Additionally, if a single tire is replaced by the full size spare tire, hopefully the new replacement tire can remain as the spare tire until the other four tires have worn out and need to be replaced.

Replacing Pairs of Tires or Individual Tires

There are several suggestions that have been offered to drivers who are replacing pairs or individual tires on their four-wheel drive and all-wheel drive vehicles. Some vehicle manufacturers recommend that all tires maintain the same rolling radius and circumference, while others suggest that all tire circumferences remain within 1/4- to 1/2-inch of each other. Other vehicle manufacturers recommend that all four tires remain within 2/32-, 3/32- or 4/32-inch of each other, or within 30% of each other in relative remaining tread depth.

Regardless of the vehicle manufacturer's recommendations, the least stressful application is when all four tires are the same tire brand, tread design and equivalent tread depth.

Before buying pairs or individual tires for all-wheel drive and four-wheel drive vehicles, drivers should read their vehicle's owners manual or contact the dealer's service department for confirmation of their specific vehicle's requirements.

Matching Tires By Shaving Them to Maintain Equivalent Tire Tread Depths

What does a driver do if one tire has to be removed from service when it and the other three tires have already worn to two-thirds to one-half of their original tread depth? Simply installing one new tire runs the risk of drivability problems or expensive driveline damage. Replacing the other three partially worn tires along with the damaged tire significantly increases the cost.

The Tire Rack can provide a solution by matching the tread depth of the replacement tire to the tread depth of the partially worn tires that will remain on the vehicle by removing tread rubber from a new tire on a specialized machine that operates as a tire lathe. While this may seem counterintuitive, the value of the mileage sacrificed by the one replacement tire is considerably less than the cost of rebuilding worn driveline components.

The Tire Rack has offered a tire shaving service that has been primarily used for preparing competition tires for racetrack use. This same service can also be used to remove tread rubber from new pairs or individual street tires used on four-wheel drive and all-wheel drive vehicles to allow them to match the remaining tread depth of the other partially worn tires that will remain on the vehicle. In addition to providing equivalent tread depth to eliminate driveline stress, shaved tires will also better match the traction and handling qualities of the remaining worn tires.

While the cost of our street tire shaving service will range from $25 to $35 for each tire, it is significantly less than the cost of unnecessarily replacing the remaining two or three good tires with lots of mileage still available from them.

Here are recommendations from some of the manufactures that the Tire Rack currently serves for matching the tires used on their four-wheel drive and all-wheel drive vehicles. Additional recommendations from other Original Equipment Vehicle Manufacturers is pending.
Audi As published in their vehicle owner's manual, "rolling radius of all 4 tires must remain the same" or within 4/32-inch of each other in remaining tread depth.
Porsche Cayenne within 30% of the other tire on the same axle's remaining treadwear.
Subaru Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.
I dont think there should be any questions now :D

Pick some good tires.
 

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Discussion Starter #4
wow I quess i better go back and just pay the extra money for the front ones. thanks for the advice. I appreciate it
 

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That is the only reason I am okay with my car being FWD. AS much as I would love AWD, I only have to replace two at a time. I was in a bind a few months ago when I got my car I had no cash but the wires were showing through on my front two tires. Too bad I still need an alignment though because now they are already all screwed up.
 

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Not really a "definite", but it does cause excess wear on the diffs and t-case, which could result in failure.
 

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While it's a good idea to replace all 4 I don't think it's necessary (personally speaking). I've never seen or heard of anyone changing just 2 tires on an AWD and breaking something.
 

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Does that mean that if you blow a tire and need to use the spare you in shit !?
Spare should be the same diameter. Granted if you change wheel diameter then you can look into a new spare tire. But you shouldn't be driving around daily on a spare.

I personally always do 4 tires. But with regular rotating and proper alignment, mine wear out evenly.
 

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I call hosh posh on replacing all four tires. It may (on a small scale) cause wear, but at the rate these cars diminish, other drivetrain parts will be due for a change.

I could see it doing damage if all four tires were already bald, and you put one new one on. But if you are still on decent tread, throw a fresh one on if need be.

At the very least, change them in pairs if you feel sketch about it.

If you want to get anal about the situation, measure the roll out of the new tire(s). If they are close, run em.

http://www.mickeythompsontires.com/tech.php?bulletin=s8
 

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Its bullshit. Your tires are different diameters all the time from bumps, heating/cooling expansion effects from accelerating and weight getting distributed differently during braking, ect....

There are so many factors that change the effective diameter of tires during normal operation that if a transfer case cant stay together because of 1/32" difference then we all got bigger problems.

Not to mention, you guys really think tires have such great quality control that they all roll out the same diameter?

I dont know, I dont think 4 less or extra rotations a mile are really any different than typical city driving if not even less.....
 

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I call hosh posh on replacing all four tires. It may (on a small scale) cause wear, but at the rate these cars diminish, other drivetrain parts will be due for a change.

I could see it doing damage if all four tires were already bald, and you put one new one on. But if you are still on decent tread, throw a fresh one on if need be.

At the very least, change them in pairs if you feel sketch about it.

If you want to get anal about the situation, measure the roll out of the new tire(s). If they are close, run em.

http://www.mickeythompsontires.com/tech.php?bulletin=s8

^This.
 

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I made the mistake of only replacing 2 on my 3000gt vr4... Tore the hell out of my vcu and took me 3 months to find a decent replacement(hence the reason I left the 3/s world... replacement parts are slim to none)... I'd replace all 4
 
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