Wheel Question...

[95 TsiAWD]

I have a 97 Mitsu Eclipse and I was planning on upgrading my wheels but I'm stuck between two sizes.

Can anyone tell me the pros and cons about the following setups ?

17 x 7.5 wheels with 235/45/17 tires VS. 18 x 7.5 wheels with 225/40/18 tires

The car is daily driven, but my track visits are often (2 times a month during the season)

 

[Templisk]

Tire size differences:
When compared to the stock GSX tires (215/50/17)
Your first choice of 235/45/17 the speedo will be 0.7% too fast
Second choice: 225/40/18 the speedo will be 1.5% too fast

Not sure if speedometer accuracy is a concern to you or not.
http://www.miata.net/garage/tirecalc.html

Weight difference: regardless of your tire sizes, you'll have more metal and more weight with an 18 inch rim. In addition you'll have more rubber. So rotating mass will go up, and in any type of event you want your car as light as possible, with as little rotating mass - unless you're using a big brake upgrade that would make it necessary it's probably best to stick to a lower size.

With that said *I* would go with the 17s, but that's a personal opinion. My reasoning is based completely on the weight issue, and running the widest tire I possibly can given the two choices.