Bigger wheels and your speedo... - BMW Forum - BimmerWerkz.com
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#1 (permalink) Old 06-06-2003, 10:10 PM
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I used to run 16" wheels stock with 225/40/16 tires

I switched to 17" wheels running 225/40/17 tires

Now I'm running 19" wheels on 235/35/19

How much of a difference will this make on my speedometer?

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#2 (permalink) Old 06-06-2003, 10:52 PM
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not much at all. i dont remember the exact number.
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#3 (permalink) Old 06-07-2003, 12:25 AM
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i'm sure theres a simple (or not so simple) way to figure it out mathmatically, but the speed is clocked at the differential so there is deffinately some change in accuracy
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#4 (permalink) Old 06-07-2003, 02:25 AM
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im gonna say >2 but <5 mph.

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#5 (permalink) Old 06-07-2003, 02:45 PM
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if memory serves me correctly about 10% plus!
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#6 (permalink) Old 06-07-2003, 04:45 PM
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uhh...itll be pretty close....
 
#7 (permalink) Old 06-08-2003, 12:57 PM
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what do you mean "difference on my speedometer" like you can go faster now?

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#8 (permalink) Old 06-08-2003, 02:34 PM
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Quote:
Originally posted by donutchow@Jun 8 2003, 10:57 AM
what do you mean "difference on my speedometer" like you can go faster now?
he means the accuracy. like if it reads 75mph he may be going around 80 becuase of his bigger wheels
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#9 (permalink) Old 06-08-2003, 02:48 PM
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i remember it was like +/- 3%.
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#10 (permalink) Old 06-08-2003, 03:44 PM
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Quote:
Originally posted by MrM3+Jun 8 2003, 07:34 PM--></span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (MrM3 @ Jun 8 2003, 07:34 PM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin--donutchow@Jun 8 2003, 10:57 AM
what do you mean "difference on my speedometer" like you can go faster now?
he means the accuracy. like if it reads 75mph he may be going around 80 becuase of his bigger wheels [/b][/quote]
yep thats right....

sorry i was in a rush and didnt have time to type....

i went from a 13" to a 17" wheel on my previous car..... although there was a 4" difference in wheel size there wasnt much difference in the profile....

if i remember i will look it up for you, if not heres how you work out the height of the wheel and tyre.... (its been a while since i have done this with out my referance chart)

so a 235/35x19.....

ok ...

235 is the width of the tyres tread in milimeters...

35 is 35% of the tyres width (= 35% of 235mm) which is the side wall height!

you then convert that figure (35% of 235mm) into inches... and then X2.

you then add that to the wheel size ? +19"

which gives you the overall height of the wheel and tyre.....

SH*T i carnt remember how you then work out the overall diameter!...... i tell you what i will get my book tommorow.... from memory its about 10%= speedo says 50mph you are actually doing 55mph.....
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#11 (permalink) Old 06-08-2003, 04:07 PM
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the "35" is the percent of the width (235) that would equal the height of the sidwall...so take 19", convert it to mm, take 235*.35 add it to the 19" converted to mm, and thats the overall hiehgt, do it for the orignal set
 
#12 (permalink) Old 06-08-2003, 04:12 PM
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Quote:
Originally posted by SpeedsterBMW@Jun 8 2003, 09:07 PM
the "35" is the percent of the width (235) that would equal the height of the sidwall...so take 19", convert it to mm, take 235*.35 add it to the 19" converted to mm, and thats the overall hiehgt, do it for the orignal set
or you could do it that way.......
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#13 (permalink) Old 06-12-2003, 06:50 AM
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BMW already has a huge problem with speedo accuracy. i think the margin is like 8% . I think that changing wheel size will be ok as long as the overall size of the rim and tire are the same as the stock ones. but if they are too big , the speedo will show you going slower than you are actually going.

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#14 (permalink) Old 06-13-2003, 11:08 PM
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the way you guys are finding it is wrong... you are taking the tire width or the wheel size (19" 17" etc) but you have to find the circumference to get the rolling distance.

Step 1: finding total diameter - 19" x 1.35 (35%) = 25.65"
Step 2: finding circumfrence - 25.65 x 3.14 = 80.541" in 1 wheel rotation


so you do that for the old wheel and the new wheel. that gives you the difference. then do ratios of inches to a mile.

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#15 (permalink) Old 06-14-2003, 07:28 AM
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Quote:
Originally posted by Fox@Jun 14 2003, 12:08 AM
the way you guys are finding it is wrong... you are taking the tire width or the wheel size (19" 17" etc) but you have to find the circumference to get the rolling distance.

Step 1: finding total diameter - 19" x 1.35 (35%) = 25.65"
Step 2: finding circumfrence - 25.65 x 3.14 = 80.541" in 1 wheel rotation


so you do that for the old wheel and the new wheel. that gives you the difference. then do ratios of inches to a mile.
35% is .35 btw....and that 35 sidewall is the percent in height of the width of the tire, in his case 235....so in his case his sidewall is 82.25mm......id convert that into inches, and add that onto the height of the wheel....
 
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