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I just recently bought a 2010 civic. I also bought 18 inch rims to go on from summit racing and after putting them on my speedometer says I'm going about 20-30mph over what I'm actually going. I'm wondering if there is anyway to fix this without having to take back the rims I just bought.

 
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The only way to alleviate that is to get a new tire with as close to the same overall diameter as the oem tire.

use this to find out what tire size you should go with
http://miata.net/garage/tirecalc.html

for example, my oem tire size was 185/65-15, and the closest 17" tire would be a 215/45-17, which would make my speedometer read .6% slower than my actual speed (with a 60mph indicated speed, I would actually be traveling at 60.4mph)

 
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Alright thanks for your help

 
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So apparently the tires I got were the exact same diameter as the original tires on there. Now what :/

 
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Wtf? That's hella weird.. I'm out of ideas at the moment. As long as you got the correct tires for it, it shouldn't be off by anything more than 3mph or so..

 
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You more than likely went to a dealer who doesn't understand things about your cars TPMS system.
All 4 tires have to be reset to there parimeters and because of new wheels oversized tires and the lot-needing to have new TPMS installed.
Did they do this for you at your tire shop?
If you don't know...you'd better go and find out..Extra cost on that service would usually have added another $150 to $250 to your Wheel And Tire Bill.

The Italian

 
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Um.. OP didn't say anything about tpms.. And you should be able to swap them over to the new wheels. Unless I'm mistaken, 30psi of air pressure in a tire on a 15" rim (or whatever is stock) would be the same pressure as 30psi in a tire on an 18" rim. Unless I'm thoroughly confused about how pressure works or what you're talking about.

 
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Well now my speedometer says km/h instead of mph. How do I change that back haha omg

 
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Nvm about that found that button I'm dumb. It seems better now. Maybe not perfect but better.

 
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So it was reading KPH and u thought u were going faster? They should keep that button where it's not easy to get to. How often does one change it anyways.