I've been visiting these forums for a few weeks now, after searching for a SCION webbase after I sat in one at the Autoshow down here in Miami. I plan on getting a scion xA when they arrive down here.

Anyways, I'm trying to decide what I want to do with rims (stock, optional alloys, or some bigger 18's)

Now, How can I look into rims and be safe that they won't mess up my spedometer when I slap them on the car? I'm assuming if I get the TRD 18's, the speedo will be fine? Or will they Adjust it when they put them on? Exactly how does this stuff work? I want to get a nice set of rims without much hassle.

Thanx,
AdaM

Check here:

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

You need to worry about the overall diameter of tire with wheel. You can calculate the overall diameter of the OEM tire (180/60-15). Whatever you get should be very close to the OEM diameter. OEM is approximately 23.74" in diameter. As long as the new diameter is within +/- 3% of the OEM, you wouldn't have any issues with speedometer. I came up with following calculation.

Width AR Wheel Diameter %
185 60 15 23.7401622
195 50 15 22.6771695 -4.48%
195 55 15 23.44488645 -1.24%
205 50 15 23.0708705 -2.82%
205 55 15 23.87795755 0.58%
205 50 16 24.0708705 1.39%
205 45 16 23.26378345 -2.01%
215 40 16 22.7716572 -4.08%
205 40 17 23.4566964 -1.19%
215 40 17 23.7716572 0.13%

There are may helpful Website to do this.

That's assuming the speedo is based on the 15s. If it's similar to the echo, the speedo measures are actually based on the smaller 175/65R14 tire size, not the 15" option.

It looks SCION and Echo are slightly different even though they share the same engine and suspension components. In Japan, Echo (called Patz) is on 175/65-14 while iST (Japanese name for SCION) is on 185/65-15. I personally visited Toyota dealer in Yokohama to check this out. In California, OEM is 185/60-15.

Ok.. wasn't sure on the OEM tire specs.. you CAN get OEM 15s on the echo as well, but the speedo isn't calibrated for them.

I wonder if the gear ratios are altered to push the different OEM tires and the added weight..