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2005 Forester Sti
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364 Posts
I found them too small for the arches, they didnt look right and the ride comfort is worse. Pay the little bit extra and use the correct 245/45/18 would be my advice (which I followed when I changed mine last year)
 

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2004 FSTi Manual
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69 Posts
I’ve run 225/40/18 with no issues on coilovers and slightly lowered, ride comfort not that noticeable. I have also switched over to 225/45/18 which I prefer overall though you do pay extra.
 

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2006 Forester XTE 5 speed manual
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281 Posts
I wouldn’t.

245/45r18 to 225/40r18 would give you a big difference in sidewall height (around 20% I would guess). That would significantly alter the diameter, throw out the speedo accuracy and mess with gearing. Aside from the ride, it could also lose you grip on poor roads (not that we have any of those on the UK :shake: ).

Pretty sure it wouldn’t look great either.

I put the old tyres off my Forester on the “dog car” (Toyota). Went from 225/60 r17 to 215/55 r17. A smaller difference, but looks stupid and messed with the gearing and ride. Luckily, it only goes a few miles a week and the dogs don’t care.
 

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2004 Forester STI
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308 Posts
I wouldn’t.

245/45r18 to 225/40r18 would give you a big difference in sidewall height (around 20% I would guess). That would significantly alter the diameter, throw out the speedo accuracy and mess with gearing. Aside from the ride, it could also lose you grip on poor roads (not that we have any of those on the UK :shake: ).

Pretty sure it wouldn’t look great either.

I put the old tyres off my Forester on the “dog car” (Toyota). Went from 225/60 r17 to 215/55 r17. A smaller difference, but looks stupid and messed with the gearing and ride. Luckily, it only goes a few miles a week and the dogs don’t care.
No, certainly not a 20% difference unless you're going to fit monster truck wheels!!!!!!

45 profile to 40 profile Diameter difference: -0.89" (-3.53%)

so 3.53 x pi = 11.08% difference on rolling surface so at 70mph your speedo will read 77.7mph
 

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2005 Forester Sti
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364 Posts
'Side wall height'
245 * 45% = 110.25mm Sidewall
225 * 40% = 90mm sidewall..... Which is approx 19% smaller

Its that difference that makes the tyres just look too small in the arches, even if the car is lowered.


But yes, the rolling surface 11% difference is what makes the difference on the speedo - which is useful to avoid speeding fines in this case!
 
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