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Hey guys,

Some time ago, I swapped out my stock wheels for some Racing Hart 18" C5s. I heard this would throw things off now; in terms of mileage racked,etc. Wouldn't the speedometer also be off? Their on my 01 civic coupe which were sporting 15s. So is my odometer overstating the true mileage and am I going slower than what my speedo is indicating? Thanks.

 
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Speedometer readings are dependent on tire size not rim size. So what's your tire size?

 
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If you have right size tires, the overall diameter should be the same no matter how big arethe rims.

Bigger rims, small side walls, or vic versa.

 
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Wow. Not sure what logic you followed to get to that point, but sit down before you confuse someone further.

It's simple geometry. The speed sensor for most vehicles is somewhere along the drivetrain, and measure the rotational speed (usually notated by the greek letter omega, but in this case I'll use w). If w is defined by V/r (V=velocity, r=radius), then V= w*r.

I'm sure you didn't need that Physics 218 lesson, but the point is, as radius increases, for a constant rotational speed, velocity will increase. (Think about it... bigger radius=bigger circumference=more distance per rotation)

In realistic terms, that means larger tires are going to yield a slow speedometer reading, i.e. if your speedometer says you're going 70mph, that means you could be going 73, 74, or 75 mph. Ish.

Here's the best calculator I've found for figuring out exactly what differential you're dealing with:

http://www.miata.net/garage/tirecalc.html

And in terms of fixing the problem:

http://www.blackrobotics.com/

I know if you contact Rick's (http://ricks2k.com/, an S2000 custom parts supplier) and buy it from them, they'll be happy to help you out with any questions you may have.

Hope this helps.

 
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O_O

Larger tires = slow speed reading by your logic

and i wrote:

"Speedometer readings are dependent on tire size not rim size."

Where is the confusion?

BTW i completely intended to write up the calculated speed difference from that calculator when he posted his tire size...

 
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Take a tire of size 205/45/15, for example. That means, in the US vernacular, the tire is 205 mm from one inside of the sidewall to another, 45 mm high at its highest point inside, and is intended for a 15" wheel. (Someone please correct me if I'm wrong.) If you take a tire of size 205/45/17, all the dimensions will be the same, save for the wheel it's meant for, which, some could argue would be wheel size, not tire size. But that difference could yield a difference in radius of an inch, certainly a worthwhile amount when you're talking about measurements.

I wasn't trying to make you look foolish, if that's what you got from it. But I'm sure you can see how it could be somewhat confusing.

 
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Right, but tire size usually includes all three sets of numbers as in your example 205/45/15.

The OP already mentioned he was on 18's, so if he just said 215/40 id know it was on an 18" rim.

He wants to know if his Speedometer is misstating his speed, and i want to know his tire size so i can tell him.

 
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Yeah, but my whole point is wheel size is part of the equation. I'm just saying your statement could have been a tad confusing, not wrong.

 
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Wheel size is NOT part of the equation unless you need to use it (as in your example above) to calculate the diameter of the tire. In truth as ricanswt said, only the tire size matters. Most likely his speedometer is not off at all, since that would require a larger tire then stock, which would most likely rub the wheel wells unless he lifted his civic (which I doubt he has done). I guess it's possible to put on a smaller tire then stock, but probably not with 18" wheels, since there would have to be 0 side wall. Either way the difference in the speedo would be very minor, nothing like my truck which comes stock with 28" tires, but has 34" on it.