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Hi,

If I change my current rims (205/50 R17) to the 135M's 225/40 R18 front and 255/35 R18 rear, do I have to reset my speedometer?
 

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I do'nt make mistakes.
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carrollf said:
Hi,

If I change my current rims (205/50 R17) to the 135M's 225/40 R18 front and 255/35 R18 rear, do I have to reset my speedometer?
No. Rim size isn't the issue: tire diameter basically the same for all stock sized rubber, so revolutions per mile remain the same.
 

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You can look up the diameter for the tires you'll be putting on those new rims, and see what the % difference would be. I have a track wheel / tire setup that runs about 4% faster than the current ones, so 67.6 indicated mph is really 65 mph.
 

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Indeed it is negligible.
JL
 

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carrollf said:
Hi,

If I change my current rims (205/50 R17) to the 135M's 225/40 R18 front and 255/35 R18 rear, do I have to reset my speedometer?
The difference between 205/50-17 and 225/40-18 is 0.1%. I think I've heard that up to almost 4.0% can be acceptable (although lower is better and I wouldn't go much beyond 1.0%).

Here's a tire size calculator for you to use, it's where I got my %s above...

http://www.powerdog.com/tiresize.cgi
 

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Interesting. My car came with 205/50/16, now I'm running 235/40/17 and my mph varies by 4-5 mph between my speedometer and GPS device I use. I wonder if the wheels caused the difference.

Though I sort of like my car overestimating my speed since it helps with speeding ... I think I'm going faster than I am. :D
 

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Aren't BMW speedometers already off by 2MPH from the factory? When my odometer is reading 55, I am really only going 53. I have confirmed that with a GPS and one of those radars the cops set up on the side of the road to warn you about your speed. I would like to change my tire size to make it more accurate.
 

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autobahn said:
Aren't BMW speedometers already off by 2MPH from the factory? When my odometer is reading 55, I am really only going 53. I have confirmed that with a GPS and one of those radars the cops set up on the side of the road to warn you about your speed. I would like to change my tire size to make it more accurate.
I think it's a percentage error. At lower speeds, the difference is negligible, but at higher speeds it makes a difference. Unlocking your OBC helps you determine your actual real speed I believe......the digital readout in km/h is slower than what the analog speedo reads.
 
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