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Old 11-08-2006, 12:42 PM   #1
conefree
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Default Change in tire aspect ratio affect speedometer?

Hey everybody, quick question...

According to various websites, it suggests 175/70R14 tires for my '97 Saturn SL1. Looking at some of my local options, one of the better ones appears to be a set of Goodyear 175/65R14 tires. Will this change in aspect ratio affect my speedometer/odometer?

Thanks in advance!!
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Old 11-08-2006, 01:50 PM   #2
mr18436572
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Default Re: Change in tire aspect ratio affect speedometer?

yes

btw there are better brands out there than goodyear
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Old 11-08-2006, 01:53 PM   #3
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Default Re: Change in tire aspect ratio affect speedometer?

any suggestions? And how much would the 5% change in aspect ratio affect my speedometer/odometer?
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Old 11-08-2006, 02:00 PM   #4
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Default Re: Change in tire aspect ratio affect speedometer?

Quick answer: yes, but not much.



Longer answer: Overall diameter of the tire is what will affect your gearing and speedometer. Changing just the aspect ratio will affect the diameter of the tire. It's pretty simple math to figure out the diamter of the tire (although manufacturers will vary slightly in the actual size).

First number is the width in mm. Second is the aspect ratio indicating the height of the sidewall of the tire as a percentage of the width. And the third number is the wheel diamter.

width x (2 x aspect_ratio/100) / 25.4 + wheel_diameter = tire_diameter

175/70R14 is 23.6" tall
175 x (2 x 70/100) / 25.4 + 14 = 23.6

175/65R14 is 23.0" tall
175 x (2 x 65/100) / 25.4 + 14 = 23.0

So your speedometer will read at 103% (23.6"/23.0" x 100) of your actual speed. Which is not really a big deal; about 1.7mph off at 65mph.

What you can do is get a wider tire with a smaller aspect ratio to get it closer or a narrower tire with a taller aspect ratio. 185/65R14 would put you at 23.5" tall and is a somewhat common size, 165/75R14 would be 23.7" tall but you probably won't find one.
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Old 11-08-2006, 02:02 PM   #5
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Default Re: Change in tire aspect ratio affect speedometer?

what he said ^

as far as brands, i havent had much experience with a lot of them. ive always liked bridgestone (my car has them right now and no complaints).
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Old 11-08-2006, 02:34 PM   #6
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Default Re: Change in tire aspect ratio affect speedometer?

Thanks for doing the math for me, Darcane, beat me to the punch lol. Doesn't seem like it would be enough to worry about.
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Old 11-08-2006, 03:12 PM   #7
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Default Re: Change in tire aspect ratio affect speedometer?

Quote:
Originally Posted by mr18436572
what he said ^

as far as brands, i havent had much experience with a lot of them. ive always liked bridgestone (my car has them right now and no complaints).
I like Falken and Kumho for bang-for-the-buck ratio. I'm real happy with the Toyo Proxes4 I have on my car now, but I got a good deal on them and probably wouldn't have bought them otherwise.

Mike
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Old 11-08-2006, 10:41 PM   #8
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Default Re: Change in tire aspect ratio affect speedometer?

I don't know if this is a common thing or if my car is a little odd, but the speedo in my SL1 is almost right on (I covered 5 measured km in 2m30s keeping the speedo as close to 120 km/h as I could) with 185/70R14 tires. The odometer was 3% slow as expected (indicated just over 4.8km when I covered 5). 175/65R14 will also look really small on a Saturn - my 185/70R14 look about right.
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Old 11-09-2006, 01:29 PM   #9
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Default Re: Change in tire aspect ratio affect speedometer?

175/65/14s are a wierd size special to corollas and geo prizms, mostly.

You should check their load rating as they might not take the weight of the proper saturn size. 87 is the load rating IIRC.
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Old 11-09-2006, 06:32 PM   #10
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Default Re: Change in tire aspect ratio affect speedometer?

175/70R14 is pretty unusual as well. Saturn really should have gone with 185/70R14 as OEM, they are much more common and the same diameter as the 195/60R15 used on the Sx2s.

Load index for most 175/70R14 is 84 and for 175/65R14 is usually 82, so technically the 175/65R14 are underrated, but they should be OK if you raise the inflation pressure a bit (maybe 34 front and 30 rear).
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Old 11-09-2006, 09:03 PM   #11
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Default Re: Change in tire aspect ratio affect speedometer?

I have 185/65-14's on mine and they seem to be fine. I don't like the look of smaller tires in the wheel well. As others have said Kumho's seem to be pretty good value, better than typical Goodyears in the same price range. Speedo's are often out be ~5% so it depends on how accurate it is now whether you'll really notice the difference going to a different size. I think tirerack.com has dimensional sizes for each specific tire. Tires of the same rated size sometimes have different actual dimensions.
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Old 11-09-2006, 09:28 PM   #12
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Default Re: Change in tire aspect ratio affect speedometer?

I've got 195/60/15 Toyo Spectrums on my SL and my speedo is off about 6km/h. to the - side. Maybe that's why my car feels slow..or maybe just cuz its an sohc.
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Old 11-10-2006, 12:08 PM   #13
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Default Re: Change in tire aspect ratio affect speedometer?

here use this http://www.discounttire.com/dtcs/infoTireMath.dos
tire calculator
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