My GS430 came with the 225/55 R16. I'm planning on upgrading to 245/40 R18 tires. I just need someone to verify some computations of mine.

(25.4mm = 1 inch)

225/55 R16 -- 225 x .55 = 123.75mm / 25.4mm = 4.872 inches + 16 inches = 20.872 inches total diameter (stock)

245/40 R18 -- 245 x .40 = 98mm / 25.4mm = 3.858 inches + 18 inches = 21.858 inches total diameter (upgrade)

21.858 / 20.872 = 1.04725 = 4.725% positive deviation from stock

Will this 4.725% diameter increase require a recalibration of the speedometer? Are my computations correct?

Also, for the 235/45 R17 optional tires -- 235 x .45 = 105.75mm / 25.4mm = 4.1633 inches + 17inches = 21.163 inches total diameter (optional)

Therefore, 21.163 / 20.872 = 1.01396 = 1.4% positive deviation from stock.

Someone told me that a 3% or less deviation in wheel size does not really affect the speedometer too much. How true is this?

I sure hope I didn't confuse you guys too much.

 

all I know is that my tire size on my car is 245/40/18's and my speedo is pretty accurate, give or take a mile or two, but in the long run its acurate. Ive had my freind cruise alongside of me and we both started from 50mph and gradually climing to 80mph and had cell phones to comunicate with each other. we decided on going on intervals of 10mph and from 50-60mph my speedo was pretty accurate

 

According to this calculator you are alright.... http://www.miata.net/garage/tirecalc.html

 

The speedometer only measures the front tires' rotation, right? Meaning, if I were to put bigfoot tires at the rear and stock tires in front, the speedometer should still be accurate, right?

 

IS2000, your idea is good but you've only added sidewall height once (ok if you were using radius, but you are using wheel diameter). You'll need to add it a second time to get an accurate diameter for the wheel and tire combination.

So your stock diameter is actually 25.744 in and your planned 18" diameter is 25.716 in giving a fairly close match.

 

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