Subaru Forester Owners Forum banner
1 - 13 of 13 Posts

·
Registered
2014 Forester 2.5i CVT
Joined
·
320 Posts
Discussion Starter · #1 ·
I'm considering on using wheels that are 1" in diameter smaller for the following reasons:

1. 225/55r17 wall height is what's on XT (I'm aware that XT's rim is 18" btw)
2. tires in that size are lighter, cheaper, quieter and have good reviews
3. I don't need 8.7" clearance and I'd be fine having .5" less
4. Lowering center of gravity by 12.7mm could improve handling
5. Less rotational weight could improve acceleration and braking (by about 5%)
6. Full size spare would fit in the place of the doughnut


So far, based on what I could find, it appears that 12.7mm change in radius would not affect alignment. (it would affect speedometer by 3.4%)

I read that number one concern would be scrub radius. Based on my (back of an envelope) calculations I should use rim with 3-4mm higher offset to keep the scrub radius the same (use wheels with 50-53mm offset). My calculation was based on assumption that struts are angled at about 25deg.

What I don't want is handling, alignment or tire wear problems. I see a lot of benefits in changing the wheel size and I'm curious if anyone has done (on earlier models perhaps) it and to what effect?
 

·
Registered
2014 Impreza Premium 2.0 AT w/ Manual Mode
Joined
·
267 Posts
I think it would be ok, I put 215/65/17 snow tires on mine for the winter and the tire shop said it will be fine.
 

·
Registered
2004 FXT 5mt
Joined
·
296 Posts
3.4% is just slightly too much. Tire Rack recommend 2% or less in diameter difference. What you will find is that even at 3.4% difference, not only will the ODO and Speedo be off but you will also use more gas because you've effectively increased RPM due to the smaller diameter. I know this for a fact because I ran my WRX tires/rims on my FXT and the calculated difference was 4%. On the freeway the higher RPM being turned for a given speed was very noticeable. Over a long road trip, it would no doubt turn into a measurable increase in fuel consumption.

Every tire is different and manufacturers don't all measure the same, so calculation alone won't give you the actual numbers until you measure the exact tire you're considering. I'd give Tire Rack a call and propose this to them and see what they say.
 

·
Registered
2014 Forester 2.5i CVT
Joined
·
320 Posts
Discussion Starter · #4 ·
3.4% is just slightly too much. Tire Rack recommend 2% or less in diameter difference. What you will find is that even at 3.4% difference, not only will the ODO and Speedo be off but you will also use more gas because you've effectively increased RPM due to the smaller diameter. I know this for a fact because I ran my WRX tires/rims on my FXT and the calculated difference was 4%. On the freeway the higher RPM being turned for a given speed was very noticeable. Over a long road trip, it would no doubt turn into a measurable increase in fuel consumption.

Every tire is different and manufacturers don't all measure the same, so calculation alone won't give you the actual numbers until you measure the exact tire you're considering. I'd give Tire Rack a call and propose this to them and see what they say.
Increase in RPMs is something I considered. I'd think that CVT being continuously variable should fix that since it's trying to run the engine at lowest RPMs and the top ratio is rather high - higher than XT's, Mazda's CX5 and RAV4. I calculated speed difference based on revolutions per mile as provided by tirerack (it came to 25 more revs per mile IIRC).
 

·
Registered
2014 Impreza Premium 2.0 AT w/ Manual Mode
Joined
·
267 Posts
Yeah just a little taller, clears everything so it's good to go!
 

·
Registered
2014 Forester Ltd. CVT
Joined
·
1,779 Posts
I'm considering on using wheels that are 1" in diameter smaller for the following reasons:



So far, based on what I could find, it appears that 12.7mm change in radius would not affect alignment. (it would affect speedometer by 3.4%)

I read that number one concern would be scrub radius. Based on my (back of an envelope) calculations I should use rim with 3-4mm higher offset to keep the scrub radius the same (use wheels with 50-53mm offset). My calculation was based on assumption that struts are angled at about 25deg.

What I don't want is handling, alignment or tire wear problems. I see a lot of benefits in changing the wheel size and I'm curious if anyone has done (on earlier models perhaps) it and to what effect?
I want a handling report on this... I've got a set of winters from my SH I'm thinking of throwing on, but would like to know handling difference (I could deal with the 3+% odo screw up, but want to be safe this winter).
 

·
Registered
2014 Forester XT Touring CVT
Joined
·
109 Posts
I recently installed my Winter wheels/tires. I went down one wheel size to 17" and used slightly narrower, but higher profile 215/65-17 tires. (OEM is 225/55 R18)

Using an online tire calculator this setup runs very close to the OEM tires. The calculator shows the OEM tire size gives 727 revs/mile while the Winter setup gives 720 revs/mile; just a 0.9% difference.

I opted for Michelin X-Ice XI3 snow tires and Motegi MR119 wheels. It looks sharp and the Michelin's performance on dry pavement is better than Blizzack which is what I've always used in the past. Looking forward to getting them into some snow.

 

·
Registered
2014 Forester XT Touring CVT
Joined
·
109 Posts
Congrats on the tires! Looks really cool. How did it affect ride quality and comfort?
Thanks. Ride quality is about the same as with the all 18" all season tires. The handling is a little bit more sluggish though most likely due to the greater flex from the higher profile and the deeper tread of the Winter tires.
 

·
Registered
Joined
·
33 Posts
Bump ...

I recently traded in my SH for an XT SJ, and kept my old snow wheels/tires knowing that the bolt pattern was the same. I didn't realize that Subaru had changed the diameter size going from SH to SJ until after I mounted my old winter tires.

I'm glad to know most people don't think there will be any issues, other than running slightly higher revs (roughly 30 added revolutions per mile) and adding phantom miles (roughly 40 miles added to odometer out of every 1000 miles).

I want a handling report on this... I've got a set of winters from my SH I'm thinking of throwing on, but would like to know handling difference (I could deal with the 3+% odo screw up, but want to be safe this winter).
Handling is actually improved. The slightly lower ride height is noticeable in suspension response and the much lighter wheels are noticeable during cornering and stopping. I haven't noticed the loss in clearance yet, but I'm rarely surfing deep snow in these tires as the roads get plowed so quickly where I drive. I haven't really noticed the added torque, as I'm still breaking in the vehicle.

These snow tires have only 1-2 seasons left anyways, so after they wear out I'll probably get something closer to OEM size to bring the RPMs and mileage back in line.
 
1 - 13 of 13 Posts
Top