i was always under the impression that no matter how wide your tire is, the speedo will always be accurate as long as the sidewall size stays the same but this site is telling me different, saying that if i go to a 265 width from a 245, but keep the sidewall at 65, the speedo will still be off. anyone know anything about this?
http://www.miata.net/garage/tirecalc.html
tire sizes?
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WINNIPEGS_MOST_WANTED
- Posts: 222
- Joined: Fri Jun 15, 2007 12:41 pm
- Car: 2003 Nissan Pathfinder
Re: tire sizes? (timmack)
The sidewall value is calculated as a percentage of the section width. You would have to adjust both the section width and sidewall aspect ratios to keep the ratio between the two tires the same.
245/65 is...245mm = 9.645" * .65 = 6.269"
265/65 is...265mm = 10.433" * .65 = 6.781"
... so 6.269" is the desired height divided by 10.433" = .6008
This means you would want a 265/60 tire.
245/65 is...245mm = 9.645" * .65 = 6.269"
265/65 is...265mm = 10.433" * .65 = 6.781"
... so 6.269" is the desired height divided by 10.433" = .6008
This means you would want a 265/60 tire.
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ubpsanity
- Posts: 59
- Joined: Fri Nov 30, 2007 5:20 am
- Car: 2001 Pathfinder SE, 1999 Mitsubishi Eclipse Spyder
Re: tire sizes? (WINNIPEGS_MOST_WANTED)
Just out of curiosity, is your speedo off now?
Mine was always slow by about 4-5% with the factory size tires. So if I was doing about 130 (indicated) on the 401 I was really only doing 123. (of course I never did that, that would have been illegal) So when I got new tires last year, I got a size bigger, since it was about 4% larger in diameter.
I believe the factory size is a 255/65-16, so I went with a 255/70-16, and now my speedo is pretty well perfect, calibrated by my GPS.
Mine was always slow by about 4-5% with the factory size tires. So if I was doing about 130 (indicated) on the 401 I was really only doing 123. (of course I never did that, that would have been illegal) So when I got new tires last year, I got a size bigger, since it was about 4% larger in diameter.
I believe the factory size is a 255/65-16, so I went with a 255/70-16, and now my speedo is pretty well perfect, calibrated by my GPS.
Re: tire sizes? (ubpsanity)
im not sure, never had it calibrated. if it is, its not off by much. but the wheels i have apparently werent an option on the SE in '02, just the LE. when i try to size my pathfinder and i type in SE, it only gives me a 16" option but my rims are 17". weird, i would have never guessed in a million years that the width would have made a difference in the speedo.
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ubpsanity
- Posts: 59
- Joined: Fri Nov 30, 2007 5:20 am
- Car: 2001 Pathfinder SE, 1999 Mitsubishi Eclipse Spyder
Re: tire sizes? (timmack)
the width doesn't make a difference to the speedo, it's the height that makes the difference.
if you use a 265/65-17 tire, the height is 30.6 inches, but if you change the tire to a 245/65-17 tire the height is now only 29.5 inches.. however if we went with a 245/70-17 size tire, we get 30.5 inches. Which is practically the same as the original. What happened here is that we used a narrower tire in both cases, but different profiles. the 65 and 70 is the percentage of the width to the height. so the sidewall is taller with the 70 series tire. 6.8 inches tall with the 70 series, but only 6.3 with the 65 series.
Now we get into confusing metric with imperial measurements. the 265 and 245 are in millimetres. but on a 17" tire..
play around wth the calculator you found, it's a great toy. I've had it bookmarked for a while.
if you use a 265/65-17 tire, the height is 30.6 inches, but if you change the tire to a 245/65-17 tire the height is now only 29.5 inches.. however if we went with a 245/70-17 size tire, we get 30.5 inches. Which is practically the same as the original. What happened here is that we used a narrower tire in both cases, but different profiles. the 65 and 70 is the percentage of the width to the height. so the sidewall is taller with the 70 series tire. 6.8 inches tall with the 70 series, but only 6.3 with the 65 series.
Now we get into confusing metric with imperial measurements. the 265 and 245 are in millimetres. but on a 17" tire..
play around wth the calculator you found, it's a great toy. I've had it bookmarked for a while.
Re: tire sizes? (ubpsanity)
ahh that explains it, i always thought that the sidewall number was in a form of measurement, i wasnt aware that it was a percentage of the width, and by changing the width, you automatically change the height even though the number stays the same. thanks for putting that in perspective. and yea i agree, that tire calculator is a pretty handy tool.