"john" > wrote in message ...
> My '03 Toyota Matrix XR has 17" stock wheels, which apparently came with its
> "Sports Package"

The biggest thing to be careful of is will the 16" wheels fit? If the "sports package"
came with bigger brakes (or if the previous owner installed them) the larger wheels
may be required.

When thinking about speedometer calibration issues, don't look at rim diameter
(16" vs 17"), look at the actual size of the tires. Compare radius, diameter, or
circumfrence, but be clear about what you're doing.
Tire radius = (wheel diameter/2) + tire width*(aspect ratio).

Of course you will need to convert to the same units--our wheels are measured in
inches, and the tire width is measured in mm.

Here's an example, a 225/50-17 ti
(17"/2)(25.4mm/1") + 225mm(.5) = 328mm tire radius.

Also speedometers typically read a little too fast. It's common to have a speedometer
read 67 or 68 when you're going 65. If you check yours before changing the tires,
and consider how much larger or smaller the radius of the new tires is you'll know
exactly how the tire size change will affect the speedometer reading.