Mark40 Posted October 28, 2017 Share Posted October 28, 2017 (edited) I've read that the tread depth of the tires on our symmetrical all wheel drive Subarus need to be pretty much equal. We suffered a damaged tire and received a replacement. Existing tires are 7/32 and the replacement is 9/32 Thanks. Edited October 28, 2017 by Mark40 Quote Link to comment Share on other sites More sharing options...
987687 Posted October 28, 2017 Share Posted October 28, 2017 Since I have no idea what car you have, I'll assume for the sake of math that you have 205-55-16" tires, a pretty common subaru size. Subaru specifies 1/4" difference in circumference on most vehicles. Let's say the tire with 9/32 is brand new at the nominal diameter of 24.88 inches. Its circumference is then 24.88*pi=78.16" The tire that's 7/32 is actually 4/32 smaller diameter, and has a circumference of 77.77". The difference in circumference here is 3/8", just a bit over the specified 1/4". Of course if you have an outback or forester, you have a larger tire and thus the difference in tread has a smaller impact on circumference. I'd probably run it and not worry. I'd put it on the front where it wears faster and rotate the tires so that one always stays on the front and the rest rotate around to wear it faster. The decision to run a tire slightly out of spec is on you, though. 1 Quote Link to comment Share on other sites More sharing options...
Numbchux Posted October 28, 2017 Share Posted October 28, 2017 Subaru recommends all tires be within 3/32 tread depth assuming the exact same make and model tire to begin with. So you should be good. 1 Quote Link to comment Share on other sites More sharing options...
86BRATMAN Posted October 28, 2017 Share Posted October 28, 2017 Only subaru documentation I've seen on the matter says 1/4" circumference. 2 Quote Link to comment Share on other sites More sharing options...
Mark40 Posted October 29, 2017 Author Share Posted October 29, 2017 The tires are on a Subaru Outback, 225-55-17 and are General Altimax Quote Link to comment Share on other sites More sharing options...
987687 Posted October 29, 2017 Share Posted October 29, 2017 It's too late to do any more math , but you should be fine... Grab your calculator and go hog wild. I'd just run it. 1 Quote Link to comment Share on other sites More sharing options...
idosubaru Posted October 29, 2017 Share Posted October 29, 2017 (edited) Run it. Put it on the front and leave it there while rotating the other three and it’ll wear faster and “catch up” to the rest if your driving wears the fronts faster. I’ve had I think 3 new low mileage Altimax get bubbles on the side walls. They’re a very nice tire for the price point but i think they’re susceptible to terrible roads (ours are awful). I’ve done it a bunch of times and never had issues. Edited October 29, 2017 by idosubaru 2 Quote Link to comment Share on other sites More sharing options...
Mark40 Posted October 29, 2017 Author Share Posted October 29, 2017 Thanks for the replies folks, to the front it goes. Quote Link to comment Share on other sites More sharing options...
Numbchux Posted October 30, 2017 Share Posted October 30, 2017 Only subaru documentation I've seen on the matter says 1/4" circumference. Well, I can't seem to find any documentation at the moment. But I work at a dealership, and 3/32 is the benchmark we use. Truth be told, tread depth will have a different effect on circumference/diameter (which is truly what has an effect on rpms, which is what will put strain on the drivetrain) depending on the overall tire size, so a circumference difference is certainly more accurate. But looks like the math is pretty close on Subaru sizes... Quote Link to comment Share on other sites More sharing options...
CNY_Dave Posted October 30, 2017 Share Posted October 30, 2017 For the worry-wort, park in a big open lot, chalk all 4 tires with vertical line where they meet the ground, have someone watch as you drive forwards and count any tire having the line reach the ground 10 times, and stop with the line straight down. 0.25" in circumference is now 2.5" in rotated distance of the line, furthest 'forward' to furthest 'back'. 1 Quote Link to comment Share on other sites More sharing options...
idosubaru Posted October 30, 2017 Share Posted October 30, 2017 For the worry-wort, park in a big open lot, chalk all 4 tires with vertical line where they meet the ground, have someone watch as you drive forwards and count any tire having the line reach the ground 10 times, and stop with the line straight down. 0.25" in circumference is now 2.5" in rotated distance of the line, furthest 'forward' to furthest 'back'. that's awesome. Quote Link to comment Share on other sites More sharing options...
86BRATMAN Posted October 30, 2017 Share Posted October 30, 2017 (edited) Well, I can't seem to find any documentation at the moment. But I work at a dealership, and 3/32 is the benchmark we use. Truth be told, tread depth will have a different effect on circumference/diameter (which is truly what has an effect on rpms, which is what will put strain on the drivetrain) depending on the overall tire size, so a circumference difference is certainly more accurate. But looks like the math is pretty close on Subaru sizes... http://jdmfsm.info/Auto/Japan/Subaru/--Articles--/--Endwrench--/Site/suspension/tire.pdf Edited October 30, 2017 by 86BRATMAN Quote Link to comment Share on other sites More sharing options...
86BRATMAN Posted October 30, 2017 Share Posted October 30, 2017 I have a friend who works at one of the Subaru dealerships in NC. He has the modern equivalent of the old endwrench pdfs in his information packet from when he was hired he said he'll get me a picture of tomorrow if need be. It also states 1/4" circumference. Quote Link to comment Share on other sites More sharing options...
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