Isn't it tire diameter that effects how the ecm calculates speed? Stock tires are 215/45/18

 

There is a common misconception that the 2nd value on a tire code is that tire's actual sidewall height measurement. It does denote the tire's sidewall height, but this number is a percentage of the tire's width, not its actual height. We can calculate the sidewall height fairly easily, which is something that should be checked before buying tires. This is because changing the sidewall height throws out the speedo.

Therefore, for a 215/45R18, the tire's sidewall height is 45% of it's width, (215mm). This means that the sidewall is 96.75mm tall.

A 235/40R18 has a sidewall 94mm tall.
Total tire+wheel height difference: 2.75mm x2 (doubled for sidewall above and below the wheel) = 5.5mm

A 245/40R18 has a 98mm sidewall.
Total difference = 1.25mm x2 = 2.5mm

A 225/45R18 has a 101.25mm sidewall.
Total difference = 4.5mm x 2 = 9mm

Even though the 215/45R18 and the 225/45R18 both share a 45 series profile, the speedo would be out further with this tire (225/45R18) than it would for either a 235/40R18 or a 245/40R18.

1010 Tires has a good calculator to tell you the speedo difference.

http://www.1010tires.com/tiresizecalculator.asp

 

its a little more than your 500.00 limit, but I just went with some 18lb wheels (HD Tuning, Clutch, Bronze $185.00 each)

its helped the feel of the car in the corners quite a bit...