New Tire Time!

2758 Views 12 Replies 8 Participants Last post by KLEMMER, Dec 15, 2018

Jackchickadee

Discussion Starter · Dec 4, 2018

Hey everyone,

This isn’t specifically a Subie question, but I trust you guys and can’t find a straight answer with a cursory google search:

I’m currently running 17x7.5 wheels on my 2016 Forester along with the stock Yoko tires (225/60/17). It’s almost time to replace them but before I do I want to make sure that I can get my speedo back to normal. Regardless of what willtheyfit.com says, increasing the wheel width threw off my speedo by 2-3 mph high. I want to take this opportunity to fix that.

So what does the hive mind recommend? I’m looking for something like the Geolander A/T’s, as most of my driving is in town and highway in Phoenix with a few significant snow/trail adventures throughout the year.

Thanks in advance. You all rock.

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cbrown165

· #2 · Dec 4, 2018

I’m confused. Increasing the wheel width should have zero impact on the speedometer of the car........only change in wheel circumference(or height diameter) would effect the speedometer......right?

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Nuke8401

· #3 · Dec 4, 2018

Jackchickadee said:
Hey everyone,

This isn’t specifically a Subie question, but I trust you guys and can’t find a straight answer with a cursory google search:

I’m currently running 17x7.5 wheels on my 2016 Forester along with the stock Yoko tires (225/60/17). It’s almost time to replace them but before I do I want to make sure that I can get my speedo back to normal. Regardless of what willtheyfit.com says, increasing the wheel width threw off my speedo by 2-3 mph high. I want to take this opportunity to fix that.

So what does the hive mind recommend? I’m looking for something like the Geolander A/T’s, as most of my driving is in town and highway in Phoenix with a few significant snow/trail adventures throughout the year.

Thanks in advance. You all rock.

How about speed by gps. Something isn't right, same tires on different rims won't change tire dia.

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FozzieBalou

· #4 · Dec 5, 2018

cbrown165 said:
I’m confused. Increasing the wheel width should have zero impact on the speedometer of the car........only change in wheel circumference(or height diameter) would effect the speedometer......right?

Not exactly 100% true.

From what I remember from all of my tire searches if you increase the width of the tire you also increase the height of the sidewall.

If our standard tire is a 225 60 R17, it means that the center opening for the rim is 17 in and the width of the tread is 225 mm and the height of the side wall between the opening and the tread [the aspect ratio] is 60% of the tread width.

So if you increase the width of the tread you're going to also increase the height of the sidewall by 60% of whatever your width increase is. This means that if you go up 10 mm in width, you are going to go up 6 mm in height of the sidewall. *** This means that the outside diameter of the tire will also increase thereby meaning it turns slower across the same distance - it turns fewer times in the same distance.

*** As a side question, I think this means the total height of the tire goes up 120% as you have two sidewall sections - in this case, you're getting taller by 12 mm total. Right?

Mind you, we're talking about a fairly small percentage change, but it still is a change.

If my math is incorrect or if I'm not thinking right, I would hope that one of the tire Specialists on this site would correct me. But as I have always been informed when you increase the width of the tire you end up increasing the overall diameter if you do not decrease your aspect ratio.

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Jackchickadee

Discussion Starter · #5 · Dec 5, 2018

How about speed by gps. Something isn't right, same tires on different rims won't change tire dia.[/QUOTE]

Basic geometry says that if you change the angle of the sidewall (in this case by widening the rim) that another measurement has to change. Since the size of the sidewall is constant, then the profile of the tire has to change, however slightly, to accommodate.

I haven’t tried speed by GPS, but the discrepancy started when I changed wheels so I assumed causation.

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Jackchickadee

Discussion Starter · #6 · Dec 5, 2018

Basic geometry says that if you change the angle of the sidewall (in this case by widening the rim) that another measurement has to change. Since the size of the sidewall is constant, then the profile of the tire has to change, however slightly, to accommodate.
I haven’t tried speed by GPS, but the discrepancy started when I changed wheels so I assumed causation.

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Jackchickadee

Discussion Starter · #7 · Dec 5, 2018

You are correct, to a certain extent. The max diameter of a tire should be when the wheel width and tread width match. Anything else increases the angle of the sidewall, decreasing the total diameter of the tire and wheel.

We’re talking small numbers here, but at highway speeds they should still add up.

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Fishinfool

· #8 · Dec 6, 2018

Replace them with anything. Chiseled rock is better than the factory tires. I have 225 65 17 geolandar ATs G015s and simply ignore the minor discrepancy in speed as do most.

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Jackchickadee

Discussion Starter · #9 · Dec 7, 2018

Fishinfool said:
Replace them with anything. Chiseled rock is better than the factory tires. I have 225 65 17 geolandar ATs G015s and simply ignore the minor discrepancy in speed as do most.

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I know I know. I’m contemplating those (if I can find them anywhere) but the speed discrepancy drives me nuts, hehe.

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Jackchickadee

Discussion Starter · #10 · Dec 13, 2018

So just to close this bad boy out, I ended up going with the Yoko Geolander A/T G015s. According to GPS speedo, the 235/65/17s paired with 7.5x17s are perfect!

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Womprat57

· #11 · Dec 14, 2018

Good choice! Run 15's on the Foz and 17's on the Crosstrek, love them!

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Fourboots

· #12 · Dec 15, 2018

You know your speedo probably isn’t accurate. Mine read 2mph high and, now with geolandar g015 265/65/17 my speedo is accurate at 70 😄

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KLEMMER

· #13 · Dec 15, 2018

Jackchickadee said:
So just to close this bad boy out, I ended up going with the Yoko Geolander A/T G015s. According to GPS speedo, the 235/65/17s paired with 7.5x17s are perfect!

Great choice I'm on my second set of the G015s on my XT with the 18" wheels. Not only a great summer tire the G015s handle the winter as well. The only thing I noticed is they felt a little soft and I went +2 psi on both front and rear. I was a little disappointed in the thread life but the overall performance outweighed the treadware issue. I am also really hard on tire tread. I didn't see any reason to go with the standard "All season" tire and the G015s look great as well.

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