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      09-09-2012, 12:25 AM   #1
mark896
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Explain how going from a 35 to a 30 height doesn't change your speedo

I see a lot of owners going from a 265/35/19 to, say, a 285/30/19 in the rear and nobody is mentioning a speedo difference. Isn't the wheel turning faster if the tire is shorter?
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      09-09-2012, 12:41 AM   #2
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Yes the wheel is turning faster. However, I believe people do not mention it because the sizes are very similar and yield an insignificant difference. Using simple math and PSS 265/35 and 295/30, you can see on tirerack that the revs per mile are 790 and 799 respectively. The different is .3 inches, with the 295 still being smaller. Relatively speaking, its only off by like 1%, so at 80 miles per hour, you lose like 1 mph. Add that to the fact our speedos (not sure about the digital one) are off by like 2 or 3 as it is, the difference the minimal at best. I also remember reading somewhere that our cars don't use wheel rotation to judge speed but something else, but my memory about that is foggy at best lol so I'm probably wrong about that.

Hope this is a good enough explanation

EDIT: Also, I remember another thread on the forum talking about speedo inaccuracy, with people going like 150mph on the dash, but like 146 in reality.
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      09-09-2012, 09:59 AM   #3
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Quote:
Originally Posted by C|3R1C
Yes the wheel is turning faster. However, I believe people do not mention it because the sizes are very similar and yield an insignificant difference. Using simple math and PSS 265/35 and 295/30, you can see on tirerack that the revs per mile are 790 and 799 respectively. The different is .3 inches, with the 295 still being smaller. Relatively speaking, its only off by like 1%, so at 80 miles per hour, you lose like 1 mph. Add that to the fact our speedos (not sure about the digital one) are off by like 2 or 3 as it is, the difference the minimal at best. I also remember reading somewhere that our cars don't use wheel rotation to judge speed but something else, but my memory about that is foggy at best lol so I'm probably wrong about that.

Hope this is a good enough explanation

EDIT: Also, I remember another thread on the forum talking about speedo inaccuracy, with people going like 150mph on the dash, but like 146 in reality.
Good enough, thanks much.
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      09-09-2012, 10:24 AM   #4
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I'm considering switching to a 275/295 PSS setup soon, and I'm guessing the same logic applies to the odometer? If I use a slightly less tall tire, will the mileage be innacurate as well (miles will accrue slightly faster since the wheels are spinning faster)?

This is my only hesitation in changing to this setup versus a 255/275 setup as I'd hate to have overstated mileage when it comes time to sell.
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      09-10-2012, 04:08 PM   #5
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What matters is the overall rolling diameter. If the new tire changes the overall diameter, it will affect the speedo and odometer.

OP, you understand how the aspect ratio works right? 35*265 mm width = 92.75 mm sidewall height. 30*285 mm width = 85.5 mm sidewall height. The sidewall is shorter so the overall diameter is smaller so the tire must spin faster.
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      09-10-2012, 04:34 PM   #6
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Quote:
Originally Posted by M3PO View Post
What matters is the overall rolling diameter. If the new tire changes the overall diameter, it will affect the speedo and odometer.

OP, you understand how the aspect ratio works right? 35*265 mm width = 92.75 mm sidewall height. 30*285 mm width = 85.5 mm sidewall height. The sidewall is shorter so the overall diameter is smaller so the tire must spin faster.
Yea I understand that, that's why I was so puzzled as how everyone could change their sidewall heights but nobody was talking about their speedometers.
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