Ozark Mountain Region SCCA

Need to know what class this vehicle would be. Only modifications are 18" wheels (factory 16") and a different muffler.

STX?

F Street Modified?

Thanks

There's no such thing as "F Street Modified"

The muffler doesn't matter for F Street, but the wheels do. You can only change diameter by one inch (assuming wheel offset meets rules requirements of no more than ¼ inch different from stock)
Anyhow, it'd go in STX via the catchall section, assuming you meet the class' wheel and tire width requirements (9in and 265mm for 2WD) and treadwear rating of 200 or greater.

The rulebook can be found here.

http://www.scca.com/pages/solo-cars-and-rules

You'll be tested Sunday. j/k . It's a little unwieldy for the uninitiated.
Seriously, though, download the pdf and review the sections pertinent to you. Nobody wants to get caught out by having inadvertently misclassed their car.
See you Sunday