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Discussion Starter #1
What is up AV6ers! Really appreciate the help you guys give. I've been on this forum researching for a few months now and you guys give some awesome information.

I'm in the market for rims. Decided that I will most likely get 18s cuz the general consensus is that it looks better. My question is: I've been studying the tire size calculator at http://www.miata.net/garage/tirecalcold.html and my stock wheel size is 205/65-15. The popular tire size for our car is 225/40-18. But this gives a -1.6% difference. What's the deal? :confused:

And if I had gotten 225/40-18, would there be any strain to the engine (possibly gear ratios?) other than the speedometer being off since the diameter is smaller than my stock? Or possibly any other long term effects?
 

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i think as long as your within 3 percent you'll be alright. I used the tire calculator on 1010tires when I picked out my tires.
 

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Discussion Starter #3
hey thanx for the reply.

Nice, i like the 1010tire calculator because it actually shows how much speed (mph) i'm off with different sized tires.

So if the speedometer is off, does this also mean that the mileage is off also? or am i just over analyzing this or being naive? I apologize if that sounds like a stupid question but its been on my mind for awhile now.
 

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yea i think your mileage will be off too. i can't think of a good way to explain in though. i just found this
From a purely geometric perspective, changing the tire diameter could effect the "Perceived Mileage" - assuming the odometer (and speedometer) tabulates the revolutions (RPMs for the speedometer), the traveled distance would be greater than the measured distance if the tire size was increased. (Just as someone would run slightly further if they ran around a track in lane two versus lane one.) Calculating the MPG would provide an error unless the odometer/speedometer was recalibrated for the different size of tire.
 

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hmm. that is what i was afraid of. I just don't understand why people still get 225/40/18 sized tires if it throws them off? I think a 235/40/18 sized tire is a better fit to retain stock tire sizes and won't affect speedometer/odometer as much. but i think 235/40 tires need at least a 18x8 rim.
 

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You will get less millage with just the added weight of the 18" rims, so the difference of diameter will not change that alot.

John
 

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can you even put a 235/18 on with out rubbing or rolling the fender? everything comes with a 40 series thats why we have them, but when the tires are done i am going to go to a 45 series.
 

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From what I've heard, an 8inch wide wheel will rub, that's why we stick to the the 225/40/18s
 

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ftmjohn said:
You will get less millage with just the added weight of the 18" rims, so the difference of diameter will not change that alot.

John
Did he mean actual milage driven or gas milage? The smaller diameter wheel/tire combo will put miles on your car quicker. Well, because your wheel is a larger diameter you will have more weight towards the edge of the tire. Also I have found that usually these performance tires weigh more than the wheel itself.
 

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andreivtec said:
Well, because your wheel is a larger diameter you will have more weight towards the edge of the tire.
Does this statement look as crazy and strange to everyone else, as it does to me?

Just wondering.
 
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