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      06-04-2010, 10:39 AM   #1
Mr. ///M3 RD
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Tire Size and Speed Difference Calculator

I figured some of you might find this interesting when changing rim and tire sizes

http://www.csgnetwork.com/speedocalibcalc.html
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      06-04-2010, 12:00 PM   #2
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the difference is never much, 1 or 2 mph. unless you go from 15" to 22" rims.
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      06-04-2010, 12:06 PM   #3
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Not so, I calculated (using the German template) as much as 0.9% from 17" to 19" since I have 19" on my car.

Memphis, you cannot alway rely on what you find on Google I prefer to rely on calculated facts
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      06-04-2010, 12:13 PM   #4
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according to your website:

if you go from stock 17" 225/45 to 255/30 on 19"s

the difference at 60mph would be 0.1229702033738 more.

so with 17" rims at 60mph you would be doing 60.1229702033738 with 19" rims

even at 150mph the diff is 0.3074255084345

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      06-04-2010, 03:12 PM   #5
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Memphis, is my math so wrong? The web site calculated the difference between the 17" and 19" as 0.9 %

So according to my interpretation it would be say at 100 MPH 109 MPH (17" OEM wheels and tires against 19" wheels and tires). What am I missing here?

Thanks for the follow up
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      06-04-2010, 03:17 PM   #6
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Further, I never ever gave this issue much thought. However, I do recall that when I drove my M3 (also with 19" wheels and tires) that the TomTom speed was always higher then my M3 speed-readout. I therefore must assume that a correction on my M3 was never made to correct for the increased wheel tire size from OEM.
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      06-04-2010, 04:47 PM   #7
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Quote:
Originally Posted by solgain@mac.com View Post
Memphis, is my math so wrong? The web site calculated the difference between the 17" and 19" as 0.9 %

So according to my interpretation it would be say at 100 MPH 109 MPH (17" OEM wheels and tires against 19" wheels and tires). What am I missing here?

Thanks for the follow up
If you increase from 17" to 19" you need to dcrease the aspect ratio (45 to 30 in the example above). If you don't do that, the diameter will change and the speedo will be off. You will go faster than indicated, and the GPS was correctly stating that. However, BMW speedo's are notoriously optimistic. Probably 2 mph at 60, allowing a 2mph increase in actual speed to actually be more accurate.
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      06-04-2010, 06:46 PM   #8
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my M3 , with 19's and the correct aspect ratio as per BMW was reading 7kmh faster then actual speed. My Z4 with factory 19" rims and factory tires reads 5kmh faster than actual speed and my 325Ci with factory 17's is dead on the speed. +/- 1kmh (all measured with two different GPS units. I started checking when everyone passed me on the highway when my cruise was set at 120, the regular speed of traffic on that road).

I drive using GPS as my speed indicator, not the speedo. I like accuracy not approximation.
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      06-04-2010, 07:54 PM   #9
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Quote:
Originally Posted by solgain@mac.com View Post
I do recall that when I drove my M3 (also with 19" wheels and tires) that the TomTom speed was always higher then my M3 speed-readout.
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Originally Posted by mcbeee View Post
my M3 , with 19's and the correct aspect ratio as per BMW was reading 7kmh faster then actual speed. My Z4 with factory 19" rims and factory tires reads 5kmh faster than actual speed and my 325Ci with factory 17's is dead on the speed. +/- 1kmh (all measured with two different GPS units. I started checking when everyone passed me on the highway when my cruise was set at 120, the regular speed of traffic on that road).
unless you guys are using dewetron GPS systems, your GPS systems are not accurate. but on the other hand, all car speedos come purposely set a mile off the true speed, they're never dead on.
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      06-05-2010, 05:47 PM   #10
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Quote:
Originally Posted by solgain@mac.com View Post
Memphis, is my math so wrong? The web site calculated the difference between the 17" and 19" as 0.9 %

So according to my interpretation it would be say at 100 MPH 109 MPH (17" OEM wheels and tires against 19" wheels and tires). What am I missing here?

Thanks for the follow up
100mph times 0,9% = 100,9 mph. You would have to have a gain of 9% to get 109 mph!

So i guess it's just the normal percent/decimal dilemma.
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      06-05-2010, 06:31 PM   #11
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Quote:
Originally Posted by SHADOW4 View Post
100mph times 0,9% = 100,9 mph. You would have to have a gain of 9% to get 109 mph!

So i guess it's just the normal percent/decimal dilemma.
Looks like a dilemma alright LOL, looks like Memphis was right all along Thanks for pointing that out Shadow
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