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2021 Forester Sport
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5 Posts
Discussion Starter · #1 ·
Hey guys . I have a 21 Sport with Stock 18" Wheels that I want to put All Terrain Tires on when I am ready to purchase a new set. My main question is will my speedometer be off any by switching to the bigger All Terrain Tires?? Not sure what size but I have see several pics and love the look of the AT Tires on the Foresters.
I'm not worried about rubbing because I will be adding a LP Adventure lift kit before the tires go on to complete the look. Any suggestions on what size I can run? I see that this size fits nice on the 18" wheel 255/55/18 . Just curious if the speedo will be off or can it be adjusted with a programmer/tuner.
Pic for reference as to what I want mine to look like.

Wheel Tire Car Vehicle Plant
 

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2021 Forester Sport
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84 Posts
Check this site for tire/wheel size comparison. They also give you the speedo deviations.


I'm in the same boat as you. I'm going with either an ADF or Subtle 1.5" kit.
 

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2014 2.5i Limited CVT
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3,683 Posts
If you do not want issues and variances with the speedometer and odometer, you would need to keep stock tire size. Any variance in the size can and will provide at least some change to the speedo accuracy.

Tire size is a formula - width in mm / sidewall height / internal opening (aka wheel size).

The sidewall height is a ratio based upon the width. If you have 225/55 right now, that means the sidewall height is 55% of the width of 225 (roughly 4.09 mm). Of course, that's times 2 as you have 2 sidewall sections (top n bottom) that add to the internal opening. If you increase (or decrease) that overall height (wheel + sidewall + sidewall) you increase (or decrease) the rolling radius of the tire and get the speedo error.

Same is true of going with a wider or narrower tire; if you change to a 235/55, then you're now at 4.27 mm per sidewall section.

Most of the time, if you're just going up one size, there shouldn't be too much of an issue - at highway speeds, you're usually looking at about 1 or 2 MPH.

Check out one of the tire size comparison tools online...
 

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This site is useful for calculating the differences in tire sizes

 

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2021 Forester Base Automatic
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344 Posts
Most cars read a little high, the first size up will give you a more accurate speedo, anything after that will read low. Not that big a deal as long as you remember to avoid speeding tickets.
 

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2021 Forester Sport
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5 Posts
Discussion Starter · #6 ·
Does any company make a tuner (not just a ODB Chip)for the Forester? one that can get into tire size? I had one when I owned a Dodge Charger and could change tire size and the speedo would be spot on.
 

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2020 Forester Sport
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153 Posts
The biggest tire you can get under the fender without major rubbing is a 235/60. On stock wheels the speedo difference is so minimal there is no point in spending money on a programmer specifically for that reason. Unless you plan on lifting your sube I wouldn't worry about that aspect of the tire.

Just my opinion.
 

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2021 Forester Sport
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Discussion Starter · #8 ·
The biggest tire you can get under the fender without major rubbing is a 235/60. On stock wheels the speedo difference is so minimal there is no point in spending money on a programmer specifically for that reason. Unless you plan on lifting your sube I wouldn't worry about that aspect of the tire.

Just my opinion.
Yes I plan on adding a 3" lift kit to it but if it's only minimal 1 or 2 mph on the speedo then I won't worry about that since I have a good radar detector.
 
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