Kbach

iTrader: (1)

What needs to be done to get the speedometer correct when changing from 17" to 18" tires?

VQT

First of all, you need to post the original size then we can tell which size is best fit.

Need to have the same diameter for the speedometer to read correctly.
There are off set of the rim to worry about also. I am still strying to understand the off set regarding fit, handling...

Then again, if you want to understand it, follow the site below and order the rime from the tire store and you are good to go.

http://www.discounttire.com/dtcs/infoTireMath.dos

Kbach

iTrader: (1)

I have stock 225/55R17 tires/rims now. I saw a set of 245/40R18 tires/rims on ebay, but that makes the speedo 3.8% slow. My question is can the electronics be programs to correct for this variance or does everyone match up tires to make it correct?

VQT

I would go for the wheel that Lexus put on their sport suspension model. I think it is 245-45-18

mikevanle

iTrader: (1)

Meh, just follow traffic.

But seriously, just Google like "tire height calculator" or "tire rpm mph" or something. Try to find a tire size that will match stock that will rotate about the same amount when traveling a mile.

I6turbo

iTrader: (1)

The infamous 'speedometer error' is really a non-issue, IMO. Unless you are switching to a crazy tall or short tire, there is very little speedometer inaccuracy from a tire/wheel change. Just do the math and get an idea of the inaccuracy, and keep it in mind. For example, if you know that you are acutally going say, 61 MPH, or 58 MPH when your speedometer indicates 60 MPH (assuming the speedometer is accurate, which it probably isn't precisely), just keep that in mind and adjust accordingly if needed.

Here's a calculator: http://www.paspeedo.com/calculator.htm
In your case, if you switched to the tires you mentioned you'd actually be traveling a couple MPH slower than indicated at 60 MPH.

Boomer01

Here is a tire calculator that I always liked. It showes you a picture of the difference in tire sizes.

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