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The Tire Rack's Tire & Wheel Forum
Use this forum to discuss anything in relation to wheels to tires to offsets.
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Old 01-14-2016, 07:03 PM   #41
Flop
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Originally Posted by trizzuth View Post
Just bumping my own thread so I can have this information later once I am all set to get tires:

Original size wheel/tire combo = Style 73's, 17 x 7 and take a 205/50/17 tire. 636.8mm overall wheel+tire diameter. Speedo at 30 = 30, at 60 = 60

Best Option for Style 71's, 17 x 8.5 = 235/45/17. 643.3 overall wheel+tire diameter. Speedo at 30 = 29.7mph, speedo at 60 = 59.39mph. Speedo error = -1.01%

Alternate option for Style 71's 17 x 8.5 = 245/40/17. 627.8mm overall wheel+tire diameter. Speedo at 30 = 30.43mph, speedo at 60 = 60.86mph. Speedo error = 1.43%
Think you may have that backwards. Smaller diameter tire (i.e. the 245/40) means you are traveling slower than indicated by the speedo. Opposite for larger diameter tire (i.e. the 235/45 tire).
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Old 01-15-2016, 08:03 AM   #42
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^^ See this, this is what I used and according to it's calcs, I don't have it backwards...

Will They Fit

Also, according to this tire rack wheel diameter calculator, here are the overall diameters of the wheels:

OE setup: 205/50/17 = 25.0787 inches
New setup: 235/45/17 = 25.326773 inches

So larger overall diameter on new setup would lead to a slight loss in mph on the speedo

If I were to use 245/40/17, that is an overall diameter of 24.716536 which is actually a smaller overall diameter than my OE setup, leading to an increase in mph in the speedo because the wheel is turning around more than the OE setup. Speed sensors for calculating MPH are at the wheels, so this seems perfectly calculated to me.
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Old 01-15-2016, 02:20 PM   #43
Flop
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Originally Posted by trizzuth View Post
^^ See this, this is what I used and according to it's calcs, I don't have it backwards...

Will They Fit

Also, according to this tire rack wheel diameter calculator, here are the overall diameters of the wheels:

OE setup: 205/50/17 = 25.0787 inches
New setup: 235/45/17 = 25.326773 inches

So larger overall diameter on new setup would lead to a slight loss in mph on the speedo

If I were to use 245/40/17, that is an overall diameter of 24.716536 which is actually a smaller overall diameter than my OE setup, leading to an increase in mph in the speedo because the wheel is turning around more than the OE setup. Speed sensors for calculating MPH are at the wheels, so this seems perfectly calculated to me.
Your speedo is calibrated to the OEM tire size, that's it. It doesn't know what size tire is on the car. All it knows is MPH in terms of revolutions per second. With the 235 tire, you will be traveling faster than what your speedo indicates. Opposite for the 245 tire.
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Old 01-21-2016, 08:33 AM   #44
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Your speedo is calibrated to the OEM tire size, that's it. It doesn't know what size tire is on the car. All it knows is MPH in terms of revolutions per second. With the 235 tire, you will be traveling faster than what your speedo indicates. Opposite for the 245 tire.
This is completely backwards.

Take a quarter and a dime, and on a piece of paper, roll the quarter one revolution and then roll the dime. Notice that it takes MORE revolutions for the smaller diameter coin (dime) to get to the same spot as 1 revolution of the quarter. Hence, a smaller overall diameter tire/wheel combo compared to the OE setup that came with my car, will show an increased mph on the speedo at the same speed as the stock setup. Overall larger diameter wheel and tire combo (rolling a half dollar compared to a quarter) will actually take less revolutions to go the same distance, and hence the speedo will show a decrease in mph on the speedo at the same speed as the stock setup. Maybe you're not considering the sidewall aspect ratio, but in one of my posts above, I calculated the overall tire + wheel diameter and it all makes perfect sense to me.
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Old 01-21-2016, 01:02 PM   #45
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This is completely backwards.

Take a quarter and a dime, and on a piece of paper, roll the quarter one revolution and then roll the dime. Notice that it takes MORE revolutions for the smaller diameter coin (dime) to get to the same spot as 1 revolution of the quarter. Hence, a smaller overall diameter tire/wheel combo compared to the OE setup that came with my car, will show an increased mph on the speedo at the same speed as the stock setup. Overall larger diameter wheel and tire combo (rolling a half dollar compared to a quarter) will actually take less revolutions to go the same distance, and hence the speedo will show a decrease in mph on the speedo at the same speed as the stock setup. Maybe you're not considering the sidewall aspect ratio, but in one of my posts above, I calculated the overall tire + wheel diameter and it all makes perfect sense to me.
Whatever mate. I tried. Why do you think 4x4s have to put a lower rear gear ratio in when they go up in tire diameter? Think about it.

Ok I'll try one last time. Wrap a piece of string around a 24" diameter tire (hypothetical OEM size) and a 26" diameter tire (new tire size). The 26" diameter tire will have a longer string. The string represents how far the vehicle will travel in one revolution. Say 100 revolutions per minute equals 25MPH, which tire went further with the speedo saying 25MPH? The 26" tire or the 24" tire? The 26" tire did. In other words, with a taller tire, you are traveling faster than what is indicated by your speedo (which was calibrated to the 24" hypothetical OEM tire size).
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Old 01-21-2016, 01:22 PM   #46
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If you're saying compare 100 revs of the 24 vs the 26, then yes, the 26 would be at a father distance once it got through it's 100 revs. But considering that both tires roll the same distance, the larger diameter tire will not make it to 100 revs, it would be less.

so if the 24" tire at 100 revs reads 25mph, and then you slap on a larger diameter tire at 26", the larger diameter tire will not be at 100 revs at 25mph, it will actually be more around, let's just say 95 revs. So less revolutions at the wheel where the speed sensor is, will yield a bit less MPH on the speedo compared to the OE setup. Let's say 24.5mph.

So, with a taller tire, going the exact same speed as the OE tire (according to a GPS), your BMW speedo will read LESS than 25mph. The internet confirms this in multiple instances, not sure why you keep disagreeing here.

Here is a completely obvious example of this, with exaggerated sizes.

OE size: 205/50/17 = Overall Diameter = 636.8mm (Speedo reading at 30mph = 30mph, Speedo reading at 60mph = 60mph)

New size: 255/45/20 = Overall Diameter = 737.5mm (Speedo reading at 30mph = 25.9mph, Speedo reading at 60mph = 51.81mph)

Notice, that in both cases, the speedo reading of the LARGER DIAMETER TIRE/WHEEL reads LESS than it did with the OE tire.

Look at the middle table here, between EXISTING and NEW for Proof.
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Old 01-21-2016, 01:44 PM   #47
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Cigars soon Mike
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Old 01-21-2016, 01:50 PM   #48
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let it warm up a lil first will ya

or maybe we should have a donut meet in the snow?
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Old 01-21-2016, 01:57 PM   #49
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nothing but a pair of style 71's spinninnnnnnnnnnnnnnnnnnnnnnnnnnnnnn they spinnin they spinnin they spinnin
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Old 01-22-2016, 01:00 PM   #50
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So, with a taller tire, going the exact same speed as the OE tire (according to a GPS), your BMW speedo will read LESS than 25mph. The internet confirms this in multiple instances, not sure why you keep disagreeing here.
I have never disagreed with that. If you read all my posts, all I have said is that you will be traveling faster (check it with a GPS speedtracker app) than what your speedo indicates with a taller tire, opposite for a shorter tire. For some reason you can't understand that. Put a pair of monster truck tires on a honda civic, and I guarantee when the speedo says 15MPH you will be gonig WAYYYY faster than that. Paging @Geo31

Last edited by Flop; 01-22-2016 at 01:02 PM.
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Old 03-16-2017, 07:09 AM   #51
lllRazorlll
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Nice wheels, post more pics of your car with them
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Old 03-16-2017, 07:32 AM   #52
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Nice wheels, post more pics of your car with them
Some pics found here.
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Old 03-16-2017, 10:20 PM   #53
trizzuth
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I have never disagreed with that. If you read all my posts, all I have said is that you will be traveling faster (check it with a GPS speedtracker app) than what your speedo indicates with a taller tire, opposite for a shorter tire. For some reason you can't understand that. Put a pair of monster truck tires on a honda civic, and I guarantee when the speedo says 15MPH you will be gonig WAYYYY faster than that. Paging @Geo31


By the way bch, Waze tells me that my Speedo is 100% on point!


Sent from my iPhone using E46Fanatics mobile app
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Old 03-20-2017, 08:15 AM   #54
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my style 71 are 225/40/18 front 255/35/18 rear, fits with a 15mm spacer front 20mm spacer rear and lowered. test fitted the 18x8.5 in the front and it fits good, just have to find a set of rears
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Old 03-20-2017, 11:33 AM   #55
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I have 245/40/17 on C2 TRmotorsports wheels 17x8.5 40 offset on stock sport supension on my 323i. My speedo seems to be fine.
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