Tire Size and AWD [Merged 5-7] diameter radius rolling different sizes size

[King Salami]

Gurus - I've got an 92 TSi AWD that I recently had to replace a tire on. All other tires are about 20% worn. Will the difference in tire diameters (due to wear; tire brand & size for the new tire is the same) cause any harm to my transfer case/AWD system?
I know wheel/tire size differences are very important on an AWD, but would 20% wear on 3 tires vs. 1 new make a difference? Since I rotated my tires, I'm experiencing an odd low-pitched "hum" or "grumble", especially when downshifting or raising off the throttle.
I've asked the guys at Discount Tire and my local Audi dealer, but they didn't seem too concerned.

[NDgsx]

It should be fine. I work at a tire shop, as long as the other tires are within 75% of their original treadwear we replace one tire and have never had a problem.

[King Salami]

Oh, I almost forgot to add: It only seems to happen above 30 mph. Any ideas if it's not related to tire diameter differences? I've got about 82,000 miles on it.

[2-0turbo]

My buddy just had the dealership fix one of his tires on his WRX. Since the car had 13,000 miles on it and this was an "on-going" issue, they replaced them all to not cause problems with the LSD's. The dealer spec was 1/4" difference allowable in circumference. More than that and it could cause problems. (According to subie).

[marntson]

ok my front tires that i bought in november are bald (i have a feeling that my fronts are toed in because they are bald on the outside.

anyway i heard from someone that with an AWD you need to have all 4 tires the same. now i really hate to buy all 4 when the rears look brand new still. is this true? or do they just have to be the same tire size?

thanks for the help.

[90AWDTalon]

Just buy two tires to match the rear and get an alignment job done.

[90AWDTalon]

Also rotate your tires from time to time.

[RipperXX]

do what the guy above said and you will be fine.

[marntson]

been working on getting an alignment for like 2 weeks now and today, the day i finally have my appointment, the mechanics air compressor takes a shit and he cant do mine. thanks for the input guys i was going to buy at LEAST 2 tires this weekend. glad to know that my tranny isnt gonna take a crap because i could only afford 2 tires.

p.s. i would have rotated the tires but after seeing the wear from 2000 miles i really didnt want to trash all 4 of em.

[spooledu1st]

Hey guy whats up. I had to change rims on my car and the new set of rims has 225 50 up front and the rears are 225 55. Could I drive around with the diffrent tires on the car or will it tear something up?

[Defiant]

Always match size front-to-rear on AWD. Not doing so puts extra load on the viscous coupler that runs power to the rear wheels, and will burn it up in short order. 50 vs. 55 isn't much, but it's still wrong.

The front end weight isn't enough to matter.

[jsigone]

I have a set of tires and rims that are on my FWD but they are 225/45/16 up front and 225/50/16 in the rear. According to a tire calculator, there is a 3.7% difference between the 2 different sizes. Can i compinsate the size with less air presure? I'd put the 225/50's in the rear and 225/45s up front. If I can compinsate, how do I test this so I don't damage the LSD?

[leet]

Tires and wheels on an AWD system MUST be the same size, front and rear.

[ARTSUPRA]

Well I have read numerous threads regarding tires, I wanted to make sure that my new tires that I would put on the back be ok, tire size back - 225/50/16, front 205/55/16

Anyways, above tires are almost identical diameter Tire1: 24.86 Tire2: 24.88

So I went to a tire shop to get the tires changed over, and the guy didn't want to do it at first because he said on AWD it will mess up differential. I told him that tires are identical and it should be ok, he said whatever...... So I got the tires changed, 205/55/16 fronts and 225/50/16 rears,

As I am driving home I got really worried if the guy was right about the differential. PLEASE I NEED A KNOWLEDGEBLE OPINION.


ART

[erc's_gsx]

yes the mechanic is correct, since u have an awd containing a center differential, you should have tires that are the same size for the front and rear. get the same size tires all around before u hurt the center differential anymore.

[NewB2dsm]

I told him that tires are identical and it should be ok

tire size back - 225/50/16, front 205/55/16

i·den·ti·cal Adj.

1: exactly alike; incapable of being perceived as different
2: being the exact same one; not any other

you should really read what you write before posting b/c anyone with a 2nd grade reading level can see that 225/50 are different from 205/55

So go tell your tire place you are wrong and have a good time buying two new tires for the ones you just bought, b/c i dought they will return them since they are mounted to the rims and you drove on them. So you just bought 6 tires for your car. I think it would be wise to lissen to a mechanic if you are not 100% sure you are right, b/c if they #### up, and you are right. THEY have to pay for it. But in your case, they told you the right thing and YOU told them to do another, so its all going to come out of your pocket.

Sorry if im an ass



edit: their is not alot of differnce in the tires since your messured them but still why get widder tires in the back since your awd? does not make sence to me. should have go the same tires all the way around. But if you are sure on the diamiter then it "might" not hurt but still not a smart move to dought the mechanic then come on her and ask if you did the right thing.

[ARTSUPRA]

I'll keep above comments in mind

Mainly I got the 2 tires for a smoking deal..........FREE, and I really needed to balance all 4 and rear tires where comletely gone. I feel I acted a little bid too fast, but oh well. I didn't phisically mesured the tires but used an online calculator which indicates that there is less then .01% difference between tires, same as if your fronts were a little more worn than backs (using the same tires).

Mainly I just wanted to hear an opinion of someone who did put a different (different diameter) tires on AWD by mistake or whatever and had problems with drive train, that's all


ART

[Blam]

my mechanic told me even the same size tires with a noticeable tread difference would be a bad idea....

[98RedGs]

Originally posted by Blam
my mechanic told me even the same size tires with a noticeable tread difference would be a bad idea....

That's true I think.
But on the other issue at hand in this thread...

YES, the mech is right it's a very bad idea to put diff tires on the car. You should honestly take those off and put back on your other tires and just wait till you can purchase two more tires like the other two that you got for free.

[turbohappy]

You might want to see what the actual difference in circumference is. The size listed and the actual size are not necessarily the same. Tirerack has this for many tires.

[Defiant]

Measure from the ground up to the center of the wheel. If the fronts and rears are the same, you _might_ get away with it. But I don't see the cost of a pair of tires balancing against the cost of a new viscous coupling. I'd not try it on my car.

[crankbender]

This is a little off subject but for everybody reading you should measure even with the same set of tires.

Because of the radically different corner weights front to back you may need to run different pressures depending on what you have in your car.

[terminaltsi]

you did the right thing by looking at overall diameter, in which case you would 99% be ok with what you have.

EXCEPT tires are not always the same size even though they are stamped the same. a 205 55 16 from brand X will be slightly different in height and width than the same size from brand Y.

so now you have a 205 55 in the front which is CLOSE to what it should be in overall diameter, but a 225 50 in the rear which may or may not be CLOSE to the OD you found online.

[Rob98GST]

I ran this exact same setup on my 91 AWD for a little over 2 years over 20k miles (sold the car two months ago).

I was a little bit worried at first. The reason I did this was because I let another DSM owner (didn't know him all to well though) borrow my stock wheels, because he bent one of his. Well, about a year later when I sold my 17" wheels, I needed them back.

Anyways, he sold his car with my freakin wheels on it!!! He did at least give me the 3 good wheels he did have. Well, he had been running 225/50/16, front 205/55/16 or the other way around because 2 of the tires were 225/50/16, and one 205/55/16. Well, I bought a wheel from the junk yard and bought a new 205/55/16.

Everything was just fine for the time I ran that setup. I even rotated the tires (225/50/16 front, 205/55/16 rear). I would be more concearned with running the spare tire if I were to ever have a flat with the AWD....

[tlnpwr]

i work @ a tire shop.... the guy was right. if u doubt it check ## owners manual. last one set i did was a volvo. it says that they should be the SAME tire BRAND, TREAD DEPTH, and TREAD STYLE. i havent checked mine yet.

[Zenja]

I'm running the same setup as you and it seems to work fine. I have different brands though. I guess it's a good idea to make sure that they're identical circumference/diameter just in case. If they're a little different you can probably compensate with different tire pressures.

[CowPimp]

Originally posted by Rob98GST

Everything was just fine for the time I ran that setup. I even rotated the tires (225/50/16 front, 205/55/16 rear). I would be more concearned with running the spare tire if I were to ever have a flat with the AWD....

I had to use the donut spare after a car accident on the way to visit someon. I had to drive 250 miles back home. My car feels just as good as it did when I bought it now that the necessary body work was taken care of and I got new tires.

[AWDPetmitsu]

I would think that the engineers designed some amount of tolerance in the center diff to account for different tread depths? All four tires will wear differently. If you say that we are supposed to rotate them, I never heard Mitsu stressing it.

[ARTSUPRA]

Derived from http://www.miata.net/garage/tirecalc.html

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
205/55-16 4.4in 12.4in 24.9in 78.2in 811 0.0%
225/50-16 4.4in 12.4in 24.9in 78.1in 811 -0.1%

[Defiant]

Originally posted by Rob98GST
I ran this exact same setup on my 91 AWD for a little over 2 years over 20k miles (sold the car two months ago).

Your customer is over in the Drivetrain forum asking about parts and failures. Something about crappy DSMs.

:laugh: