AWD and Odd Sized Tyres
You may of heard about it being a bad idea to run a different sized tyre on your Subaru, or you may have not. Either way these are the reasons why you should follow Subaru’s recommendations.
Manual 5 speed Subaru’s: First I will explain how the AWD in a Subaru applies power to the front and rear of the car. In the gearbox there is a differential, known as the centre diff. It operates just like a normal diff in a car. But instead of distributing the power to the left and right wheel while allowing differences in speed between the two, it distributes the power to the front and rear diff. See, when you go around a corner the rear wheels take a shorter route (smaller turning radius) than the front wheels. This is most obvious when you do a sharp turn on sand and look at your tracks. To assist in traction in instances such as hard acceleration around corners (lifting a single corner of the car under power) or on loose surfaces Subaru included a viscous coupling on the centre diff. We thus refer to the centre diff as a VLSD (Viscous Limited Slip Differential). This unit uses preload pressure between two plates in the diff that limits slip between the two outputs. It also gives more resistance to slip via oil being forced between these plates (using “viscous” properties of the oil). The more it slips the more oil pressure that is applied to limit slip.
Now taking that into consideration you can imagine that slip between the front and rear outputs of the centre diff (the outputs that go to the front and rear diffs of the car) could cause quite an amount of heat. When driving slow it keeps cool. So massive amounts of slipping like driving around car parks or offroad are fine. But at speed all the wheels on your car need to be very similar so they don’t cause a build up of heat in the viscous coupling. If you normally run 215/60R16’s and you then run a 215/65R16 tyre (over 3% larger diameter) with three of the originals you are causing more slip than if you were doing the tightest corner you could. In fact normal driving would incur less than a 0.1% difference front to rear. Subaru recommend less than 1/4 inch (6.3mm) in the circumference difference between tyres. To figure out the circumference multiply the diameter by pi (3.14159). Or a better option is to get out your tape measure and run it around the middle of your tyres tread.
Something you may find interesting is all tyres are not equal. My Outback came with Yokohama G900 215/60R16’s. As a spare I have a Yokohama G035 215/60R16. The G035 is 15mm larger on the circumference than the G900. Both same size written on the tyres but that is the difference you can expect between even the same brand. So even if you buy the same labeled sized tyre you still might be too far out of spec.
You may be wondering what this could do. Well in the short term a very small difference like 15mm may not do much. But over time it is constantly putting more friction on your VLSD centre diff. This will cause wear and eventually it won’t hold as strong, as in it is more likely to allow a single wheel to spin under power. With a large difference like a /65 profile tyre with /60 profile tyres you could cause too much heat at speed and fuse the viscous coupling together. This would require replacing the centre diff which is an expensive job.
Auto 4EAT Subaru’s: The automatics used in most Liberty’s, Legacy’s, Outback’s and Forester’s use a different type of differential. It is just a clutch pack. In essence they act like a front wheel drive until a front wheel slips and then the centre clutch pack tightens up and applies power to the rear diff. Because of this there is not the same problem with the autos as the manual Subaru AWDs. As the rear wheels are normally just rolling along like a FWD car, not driven, there isn’t the friction between front and rear diffs when running different sized tyres. You should still keep it similar as there is a constant application of drive to the rear, just not much, about 10% power front what I’ve read. Imagine this like you are driving with your foot slightly on the clutch pedal in a manual, you get drive, but plenty of slipping too. Subaru still recommend the same difference in tyres but a general consensus of people is you can have different wear or brand tyres of the same labeled size and not have to follow the 6.3mm – 1/4 inch Subaru recommendation.
I hope that clears up questions you may have and explains what is going on when you go against Subaru’s recommendations. It is often the case manufactures over state these things. But for me at least I am going to follow their recommendation as that VLSD is the only thing making the Subaru work like a 4WD vs a FWD offroad.