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2021 - Sport w/ Stock 18" wheels - will AT tires mess with the Speedo?

6K views 7 replies 6 participants last post by  C-64 Basic 
#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.

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#3 ·
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...
 
#7 ·
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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