Quote:
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Originally Posted by ArthurS
Question:
Carson Subaru said that once you get bigger rims and tires, that your speedometer goes off. It will show that you are going faster then you actually are and would need to be adjusted. Is this true? They said they aren't aloud to do it there either way, so that makes me feel like the aren't trying to just sell me. Have any of you with rims adjusted this?
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It has to do with the overall diameter of the wheel, not just the rim. If you go to a larger rim, but have a lower profile tire, the overall diameter changes very little. This is called "plus-sizing" your rims. If you look at the tire rack specs for each tire, the last column is the revolutions per mile (or something). As long as this number is pretty close (like +/- 10) you won't notice any issues with your speedo or odo.
When I put rims on my TBird, the overall diameter of my tires was almost an inch smaller. This gave the car better acceleration, but caused my speedo to read lower than my actualy speed. Austin did the math for me.. it was something like 70mph on the speed0 = 65 in real life, and 105 on the speedo (the speed limiter) was only 95 in real life.
I am actually planning on reducing my overall diameter when I get some autocross rims and tires, since that will help the acceleration of the car. Remember, on AWD cars, all 4 tires *must* have the same overall diameter or you'll cook a differential or two! So, no mixing and matching tires unless they're the same size!
One more thing, if you were to get bigger rims, and keep the same profile on the tire, say a 225/65/17 or something, it would simply be too big to fit under the wheel well.
One more, one more thing... Carson *sells* cars with aftermarket rims.. I saw a few WRXs with Prodrive P7s on the lot when I was in there a year+ ago for my clutch replacement, so I dunno wtf they're talking about with the "we can't do rims" thing.
edit: Oh, I get it, they can't adjust the odometer... yeah there's laws against that... the speed sensor is linked to the odometer, so "adjusting" it is against the law to prevent odometer fraud.