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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Wheels and Tires Forum Sponsored by The Tire Rack > IF I GO FROM 17" to 19" will my speedometer really be off?



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Wheel offset calculator courtesy of BimmerPost.com. You may use this calculator to calculate what tire sizes you might need if you upgrade your tires, or what wheel sizes/offsets are compatible with your car.
    Current Wheel Width  inches             Current Wheel Offset  mm.
         New Wheel Width  inches                  New Wheel Offset  mm.
 
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Original Tires Diameter (in.)
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  Difference in diameter (in.)
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      03-26-2008, 12:57 AM   #1
xpillzboyx
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IF I GO FROM 17" to 19" will my speedometer really be off?

IF I GO FROM 17" to 19" will my speedometer really be off?
i saw something on the calculator about what the speedomeer will read. i dont know how to calculate all those things but lets say im going from the stock sports ackage rims they give wit my 08 e90 to maya stms 19" staggered. will it really mess us the speedometer?
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      03-26-2008, 01:31 AM   #2
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if you choose the right wheel diameter you will have no problem
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      03-26-2008, 03:17 AM   #3
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yes, if you use the correct diameter tires, you will effectively have the same rolling diameter as your stock 17" tires. Most likely, your stock tires sizes on your E90 are 225/45/17 and 255/40/17. The most common and popular sizes for 19" tires, especially staggered sizes are 235/35/19 front and 265/30/19 rear. With these sizes the overall diameter of both the 235 and 265 are less than 2% larger than your stock tire diameter. You won't throw off your speedometer with such a marginal change. In fact, BMW speedometers are notorious for reading 5-10% over your actual speed. By up sizing your wheels and tires slightly, you'll actually have a slightly more accurate speedometer reading.
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      03-26-2008, 03:42 AM   #4
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Quote:
Originally Posted by VMRWheels View Post
yes, if you use the correct diameter tires, you will effectively have the same rolling diameter as your stock 17" tires. Most likely, your stock tires sizes on your E90 are 225/45/17 and 255/40/17. The most common and popular sizes for 19" tires, especially staggered sizes are 235/35/19 front and 265/30/19 rear. With these sizes the overall diameter of both the 235 and 265 are less than 2% larger than your stock tire diameter. You won't throw off your speedometer with such a marginal change. In fact, BMW speedometers are notorious for reading 5-10% over your actual speed. By up sizing your wheels and tires slightly, you'll actually have a slightly more accurate speedometer reading.
BMW speedometers are notorious for reading 5-10% over your actual speed.
I think that is a exaggeration. mine is under 5%.
comparison to GPS
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      03-26-2008, 10:48 AM   #5
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Okay, it is a bit of an exaggeration. But it is true, that over the range of wheel and tire combinations they offer for each car, the error can be as large as 10%, however you are correct, most cars are closer to 5%. I remember BMW posting this information in a service bulletin before.
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      03-26-2008, 11:36 AM   #6
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Quote:
Originally Posted by VMRWheels View Post
yes, if you use the correct diameter tires, you will effectively have the same rolling diameter as your stock 17" tires. Most likely, your stock tires sizes on your E90 are 225/45/17 and 255/40/17. The most common and popular sizes for 19" tires, especially staggered sizes are 235/35/19 front and 265/30/19 rear. With these sizes the overall diameter of both the 235 and 265 are less than 2% larger than your stock tire diameter. You won't throw off your speedometer with such a marginal change. In fact, BMW speedometers are notorious for reading 5-10% over your actual speed. By up sizing your wheels and tires slightly, you'll actually have a slightly more accurate speedometer reading.
When you are talking about "upsizing" your wheels and tires, you mean increasing the overall tire diameter by getting a larger diameter tire not going from 17" wheels to 19" wheels.
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      03-26-2008, 12:04 PM   #7
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Quote:
Originally Posted by DrGP View Post
When you are talking about "upsizing" your wheels and tires, you mean increasing the overall tire diameter by getting a larger diameter tire not going from 17" wheels to 19" wheels.
Yes, upsizing in regards to moving to a 19" wheel and as a result, also increasing the overall diameter of you tires slightly
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      03-26-2008, 01:15 PM   #8
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How about from 17" to 18" wheels ?
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      03-26-2008, 01:38 PM   #9
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How about from 17" to 18" wheels ?
It depends what tires you use. For example, if you take the stock 17" tire sizes of 225/45/17 and 255/40/17 tires and compare them to the equivalent 18" tire sizes of 225/40/18 and 255/35/18, you'll find the 255 18" rear tires are actually slightly smaller than the 255 17". Here are some number for you to compare:


________Overall Diameter
225/45/17-------78.44 in
255/40/17-------78.63 in

225/40/18-------78.79 in
255/35/18-------78.60 in

So, moving up in wheel sizes doesn't always mean the overall diameter of the tire will be increased. Since tires are sized in regards to aspect ratios, the relationship of circumferences of the tires through different diameters isn't a linear one.
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      03-26-2008, 02:28 PM   #10
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Quote:
Originally Posted by E_90 View Post
BMW speedometers are notorious for reading 5-10% over your actual speed.
I think that is a exaggeration. mine is under 5%.
comparison to GPS
i agree to this statement, when compared to my GPS, when my car's speedo reads 107km/h, it read exactly 100 km/h on the GPS, and i am on 19s, 235 front, 265 rears
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      03-26-2008, 03:15 PM   #11
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More simply stated, when you +1 or +2 your wheel size, you go with a lower profile tire to compensate for the increased wheel size. Your goal is to maintain as close to the factory rolling diameter.
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