I have a 2008 R320 CDI 4matic. I have not been diligent about rotating tires. The rear tires are much more worn than the fronts and require replacement (they are on the wear bars). The fronts have significantly more tread.

I called Discount Tire and inquired about buying two replacement tires (255/55 R18), and was advised that it is important to replace all four tires at the same time to avoid mismatches in tire size on a 4WD that could cause problems with the transmission. The associate suggested that he could measure diameter of the existing two good tires and compare to the new tires, such that if the differential is more than 3/32 inch between fronts and rears then I should buy four new tires rather than two.

I am a bit doubtful regarding this claim, especially given that the car includes a temporary service spare that would appear to be dramatically smaller diameter than the regular tires (although I am unsure of this assumption).

Should I be concerned about putting new tires only on the rear if the front tires have about 50% wear remaining?

 

That is true when you have the 4-matic system. You must replace all of the tires once there is a different measurement between the tires. I just went through this when I hit a curb and slit one of my tires. It set me back, $1,100, but the ride now is like a new car on my 2007 R350.

 

I really don't think the world will come to an end if you replace just the rears using the same type and make of tyre as your half-worn fronts. If it really bothers you then you might want to clarify formally from your Mercedes dealership.
Brgds

 

This was the case with a (non-Mercedes) 4WD I had back in the 90s and it was mentioned in the manual. Is it mentioned in the Mercedes R-Class manual? I had a quick look and couldn't see it mentioned. If it isn't mentioned in the Mercedes manual I would suggest that you have nothing to worry about.

 

I just bought a used 2008 c330 sport 4matic and had it inspected by my longterm mechanic. He's pretty knowledgeable, having worked on European cars for almost 20 years.

He told me one disadvantage of an AWD system was this very issue-- that replacing a damaged tire pretty much required replacing all tires (unless the damaged tire was virtually new), since the varying diameters would stress the system.

I mentioned this issue to the guy at my tire shop, who is also knowledgeable, and he said that this was a big issue some years ago with Subaru. Apparently there was a class action suit against them from aggravated Subie owners, and the company ended up building more "slack" in the powertrain to allow a greater degree of tire discrepancy.

It's true that the compact spare has a different diameter, but the concern is not that damage will occur after a short distance of driving. Rather, as my mechanic pointed out, prolonged driving on wheels with varying amounts of wear can prematurely wear out the AWD system and cause it to make a whining noise-- which not many of us would be pleased to experience.

Maybe I'll call the MB dealer and see if I can get a straight answer from them on this.

 

Vince,
As I read this thread, there appears to be a hole in the flow of logic. Two of your tires are currently badly worn. Consequently, there is a difference in the diameter of the fronts compared to the rears. This is a condition that developed over a period of time. Has anyone who sells tires said that your transmission has increasingly become more at risk because of this condition?

Assuming you replace the two worn tires with new ones, what will be the relative difference in diameters front to rear compared to the current difference?

I agree with a previous poster - contact MB to see what they have to say. Ask them whether there was a point during the timeline of development of the current state where your transmission was at risk. Ask them for the data that indicates that a delta in diameter of X produced a wear factor in the transmission of Y.

I can see where a side-to-side mismatch might be a problem. However, the increasing wear to front or rear tires is likely to occur because of a pattern of regular jackrabbit starts or driving downhill or uphill more frequently than on level ground, causing wear because of increased traction on the front or rear axles.

Are your tires worn more in the center of the tread (as in a regularly over-inflated tire) than on the outer edges? Is there a true difference in diameter? Where is the diameter being measured? Is a caliper being used as the gauge vs. a tape measure? Are both tires being measured carrying the same air pressure?

He who takes the advice of vendors to spend money will spend more money than he who gets an unbiased opinion. Our economy is such that, the more money that is spent, the more money is made by the people who profit from sales.

BTW, I am not an engineer. I studied liberal arts in college and have spent the last 41 years reading and writing government rules and regulations. One of the things I read for is logical presentation. We are conditioned to spend money. Beware of the logic when someone is asking for your credit card.

JR

 

Here's an interesting factoid, just to complicate matters.

I went to this website, which tells you the actual circumference and diameter of tires once you mount them: http://www.1010tires.com/TireSizeCal...?action=submit

The rear tires mounted on a c330 Sport wheels (245/40R17) have a circumference of 1992.3mm and a diameter of 24.97".

The front tires (225/45R17) have a circumference of 1971.5mm and a diameter of 24.71".

A few posters have said that their tire dealers told them that the rear wheel treads had to be within 3/32nd's of an inch compared to the fronts.

The specifications above show that the rear tires have a .26" greater diameter than the front tires... that's slightly more than an 8/32" differential!

If MB specs the tires this way, then there must be room for greater tire differential than the tire dealers are claiming... is this not true?

 

3/32"= .094" Tread is actual the radius while .26" different is in diameter so you have to divide by 2 =.13" radius. So the different is .13" versus .094" which not much the different (.036" or close to 1/32" not 8/32" as you thought)

I think the original poster meant the different 3/32" in diameter which nothing to do with the TREAD itself as you have mentioned in your post then I am wrong.

 

I replace two tires about 4 months before replacing the other two. I'm now another 12 months and 20,000 miles since then and I've had no issues. My R350 eats through tires so fast I can't even afford to keep putting on Michelins or Continentals... Next set will be cheaper brand.

 

rostrom, is it because the R350 is so heavy that hte tires go or is just the OEM soft ones dont last long.

 

I do think the weight of the car and the all wheel drive makes it eat tires. I can't get more than 30,000 miles out of any tire so far. I had the OEM Conti 4x4 Contact, Michelin Latitude HP Tour and now Pirelli Scorpion Zero. I rotate and check pressure religiously, but still get 30k on the nose (I am 8-9k away from replacing the Pirellis). I liked the Michelins the best, but they are pricey. The Pirellis have great handling/performance but are loud. Anyone with good experiences with any other tires?

 

when i was shopping for tires for my Murano, i the tire guy said the Hankooks were nice for the money. but i went with dunlop signatures.

 

So what's the conclusion? Replace all tires or replacing the front two tires is fine?

 

Make sure that you are using the proper load range. IIRC, 105 is specified. I went with 109.

 

If you order tires from tire rack, you can have them shaved to match your existing tires, seems like a waste, but depending on the system, it may be worth it when you only need to replace one damaged tire.

 

 

I could find no mention of this in the manual nor in any service bulletin. Has anyone contacted the dealer? Does any car company suggest this, or is it only tire dealers that want to sell you more tires? I have not found anything on a tire company site either.
I have heard of this to when I was at Costco and a woman had a blowout. They told her she had to replace all 4. So much baloney. I did long internet search and could not find any car company mentioning this. I always have my guard up with garages. The place did not even know how to look up the wheel nut torque on their chart properly. They though the R was a sedan and used the S class numbers. The R is a truck and is in a different list.. luckily the same torque..

anyway I went to the michelin tire site and asked the online chat.. they gave me these.. links. Not very informatiive


https://www.cars.com/articles/do-i-n...408.1525820369

this one references subaru and audi,
https://www.souzastireservice.com/ti...408.1525820369

https://mechanics.stackexchange.com/...408.1525820369

This one claims that honda crv looks for a 3% difference in tire rotation before engaging awd, so maybe this can be usefull:
https://honda-tech.com/forums/honda-...408.1525820369


But none reference actual auto manufacturer sources... sadly

 

Great thread Vince in NC. Two ideas on this to think about.

First, TINMan22 makes a great point that you already have tires that are different diameters. The first thing I thought was, if the tire rack guy is right, isn't your current wear mismatch already causing you 4-Matic issues (or not...)? However, it's not like you are running two completely different indicated tire sizes. And there doesn't seem to be a reference in the Merc R manual requiring all 4 tires being replaced at the same time. 4-Matic has been around for a long, long time and Merc has likely figured out by now to engineer in enough give to avoid potential issues. To me it seems like the sales person was setting you up for a sale by telling you all four tire diameters had to match by less than a tenth of an inch (3/32)!

Quite possibly he meant all tires had to have "tread depth" that were all within 3/32 of an inch. That would make much more sense and is what is suggested in the previous link. The Subaru guys amazer98 referred to seem to stick with 2/32 difference in tread depth, but then the Sube is prone to drivetrain problems due to tire size mismatch.

Efzauner's link on Audi and Sube tire matching takes it one step further by rejecting diameter and suggesting measuring and matching tire circumference. This would be the ideal solution but somewhat impractical to remove one of your good tires, let the air the air out for measuring, buy two new tires, and then have them shaved down to match your other two.

So if your trying to avoid dropping $1k on tires at the moment, how 'bout this solution.

Measure the average tread debth of the two good tires. Then go pick up a pair of decent used 255/55r18's with a similar tread debth to replace the worn ones. At least then you will have a set that are similar in diameter. Pretty good chance you have a few used tire places near you that you can work with. Call 'em first and see what they have. Also the Fleabay sellers that carry used tires usually publish the remaining tread depths of what they are selling. So if you can only find one decent tire from someone local, maybe try finding the other or even a pair on line.

If your uncomfortable using different brands in combination due to possible circumference differences between manufacturers (which I would be), shop around and find the same brand and model as the two good ones. It shouldn't be too time consuming, especially if your doing it on line. You might also try a simple Google search by entering used (your brand) 255/55 r18 and look for the same model of tire you have. Based on what's usually on fleabay, you could probably solve your 2 tire problem for $100 to $125 per tire, after mounting and balancing.

Then while your rolling on the extra 10k-20k in tire usage you just gained, save up your coin for a new set of four using this reference or this reference.

Lastly, while you have the car in the air, change your 4-matic fluid and your differential fluids. It's probably never been done, it's inexpensive preventative maintenance if you use BeVo approved aftermarket fluids, and it's relatively simple. If you know and trust the shop mounting and balancing your tires, they should be able to handle it.

Hope that helps.

 

doing more sleuthing... Subaru Crosstrek owners manual


https://www.subaru.ca/WebPage.aspx?WebPageID=16400

page 11-26 has this warning, but no specific mention of transmission/differential
having problems uploading picture.so I attached pdf of the pages.

I downloaded a 2018 Audi A4/S4 manual from here and the tire section says this about mechanical damage.
https://ownersmanuals2.com/audi/a4-s...s-manual-72224

on page 341 it has this:
"On all wheel drive vehicles, all
four wheels must be equipped
with tires that are the same brand
and have the same construction
and tread pattern so that the
drive system is not damaged by
different tire speeds. For this rea-
son, in case of emergency, only
use a spare tire that has the same
circumference as the regular
tires. "
and this
"Do not replace tires individually .
At least replace both tires on th e
same axle at the same time. "

ford 2018 expedition owners manual has no mention of dissimilar tire or wear issues other than with spare.
https://owner.ford.com/content/ford-...orer&year=2018

BMW manuals where behind VIN registry wall.

jeep Grand Cherokee owners manual
https://www.mopar.com/jeep/en-us/car...rs-manual.html
page 470 has this.. handling only no damage issues
"It is recommended to replace the two front tires or two rear
tires as a pair. Replacing just one tire can seriously affect
your vehicle’s handling. If you ever replace a wheel, make
sure that the wheel’s specifications match those of the
original wheels."


My 2011 R350 owners manual has no such warnings.

So my conclusion is that it is very car dependent..So I would be doubtful of blanket warnings. I will call my dealer too! and edit this post..

 

that is not actually true if any tire is off you can just replace one

 

Agreed Tyler,

Could not find a 2009 manual, oldest i found is 2012 and all it says is keep the axles the same...

Mike

 

dup