If I wanted to get 18x9 and 18x10 wheels for my 04, what size tires would I need? I know what the last two numbers mean, but the first one, no idea.

i.e. 245/40/18

The 40 is the "lip" size between the pavement and the rim, the 18 is the wheel's diameter. I'm assuming the 245 is the width in mm of the rim. What should I get with x9 and x10s?

 

you wouldnt want a 40mm tire height b/c that would require speedometer calibration with an 18" wheel. and a 10" wide tire can run a 305/30/18 and the 9" could run a 275/30/18