rurouni_x Posted October 20, 2008 Posted October 20, 2008 I know we Can't run different sized tires on our AWD, but what if the sizes end up being around the same circumference ? Like for example I have a used set of RT 615s 225-45-18 with 5 32nds And i was thinking of pairing them with some 245/40's. So with the 225's the side wall would be 101.25mm's minus the 3 32nd's of thread wear which adds up to about 97mm of sidewall. And with the 245's New with 8 32nds the sidewall will be 98mm only a difference of 1mm. Which is safe for the diffs right?. And plus I can further match them by changing the air pressure. Good idea or bad idea? Give me your input guys. The math adds up. Now my brain hurts:confused:
mwiener2 Posted October 20, 2008 Posted October 20, 2008 Better question is WHY would you want to do this My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons
mwiener2 Posted October 20, 2008 Posted October 20, 2008 then buy two more My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons
rurouni_x Posted October 20, 2008 Author Posted October 20, 2008 then buy two more if i buy two more in the same size it will be a bigger mismatch 100.25mm to 97mm
mwiener2 Posted October 20, 2008 Posted October 20, 2008 here's why you don't do this your 225 and 245 might be real close now, but try measuring them again at 60mph not to mention odd ride and handling charateristics My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons
mwiener2 Posted October 21, 2008 Posted October 21, 2008 cause the center of a wider tire deforms more than a skinner one My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons
Guest LGT-NY Posted October 21, 2008 Posted October 21, 2008 cause the center of a wider tire deforms more than a skinner one Troof! Don't do it man.
DiscoWagon Posted October 21, 2008 Posted October 21, 2008 Where will you put the wider tires? Front or Rear?
gianspi Posted October 21, 2008 Posted October 21, 2008 The RT-615s are MUCH wider than advertised. enough zip ties and duct tape will fix anything.
rurouni_x Posted October 21, 2008 Author Posted October 21, 2008 Where will you put the wider tires? Front or Rear? i was thinking rear
rurouni_x Posted October 22, 2008 Author Posted October 22, 2008 Alright I've decided against it. I thinking of getting Kumho Xs's or Yokohama s-Drives in 245-35-18r
rurouni_x Posted October 23, 2008 Author Posted October 23, 2008 The RT-615s are MUCH wider than advertised. yeah they are here is the 615's next to some Conti Pro contacts and the 615's are like half an inch wider. Both of them are 240-40-18 Anyone interested in the 615's I can get you a good deal on them since I will not be using them. http://i46.photobucket.com/albums/f145/rurouni_xgt/IMG00078.jpg
rurouni_x Posted October 25, 2008 Author Posted October 25, 2008 Any SoCal Peeps looking for tires? I can give you a good deal on the 615's
Brady Posted October 25, 2008 Posted October 25, 2008 Another thing to consider is that tire math isn't exact. It is always approximate, and you'll see that many tires of the same size by different manufacturers, and sometimes even the SAME manufacturer will have different values for "Revolutions per mile." This means that even though the tire is the "same size" when new, they actually have different diameters. You always want to have 4 matched tires on a Subaru.
rurouni_x Posted October 31, 2008 Author Posted October 31, 2008 Something I noticed while working on one, the GT-R runs staggered tires 255-40-20 on the front and 285-35-20 on the rears. The rolling diameter of those two sizes are almost the exact same.
NSFW Posted October 31, 2008 Posted October 31, 2008 Yeah, but that's different. The people who have posted so far know way more about this than the people who designed the GTR. I haven't done this myself, so of course I'm just speculating here: In spite of the traditional responses this question always elicits, I just really don't think this is such a bad idea, if (big if!) you do your homework. Doing the math on the tire dimensions is a good start. It'd be wise to actually measure the candidate tires first, to verify that the actual circumferences are indeed close to what the math predicts. The key figure, IMO, is how many revolutions per mile each tire has. And then of course, what's the difference between the two? At freeway speeds, how many revolutions per minute will the center diff need to absorb? If it's a small number (e.g. 2) I wouldn't worry about it at all. Hint: the owner's manual indicates how much tread depth difference is acceptable between the front and rear tires. With a little math, you ca find out how many revs/mile difference is acceptable, according to Subaru. I'd trust that number a lot more than the opinions expressed above. AWD cars tend to understeer, and putting fat tires in the rear isn't going to help matters. But if you're not the sort of person who spends a lot of time driving at the limits of tire grip, that won't make a difference to you. Except maybe in case you need some emergency swerve action, in which case you'll still be light-years ahead of all the other econoboxes and SUVs on the road. So, yeah. Do your homework, choose your tires carefully. Post pics when you're done.
dudeondacouch Posted October 31, 2008 Posted October 31, 2008 The people who have posted so far know way more about this than the people who designed the GTR. :lol:
mwiener2 Posted October 31, 2008 Posted October 31, 2008 It's really simple why the GT-R can do it and we can't... The GT-R was designed to have two different size tires!!! My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons
NSFW Posted October 31, 2008 Posted October 31, 2008 What is special about the GT-R's design that makes staggered tires work?
mwiener2 Posted October 31, 2008 Posted October 31, 2008 yeah, they may have different gearing in the front and rear diffs My Mods List (Updated 8/22/17) 2005 Outback FMT Running on Electrons
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