We don't get a ton of small tire stuff around here, but I have a question or two:

What is the biggest size still considered small tire? I am guessing its a 325 radial? What is the biggest slick typically legal in small tire? And do you have to run two different style shocks or suspension setups to go from slicks to radials?

Almost all places (except for Oklahoma, from what I understand) consider a 275/60/15 radial or a 28x10.5 non-W small tire. Some of the daily driver type classes will allow a larger metric tire, only if they're on a 17" or larger rim. My car has a 295/65/15 and has to race in big tire everywhere because it's 20mm too wide.

Radials require a different suspension setup, they like to be planted with separation and need to dead hook, and need tighter compression settings.

This in Indiana also. True 10.5 races used to be 29.5x10.5 but since the 275 classes gained popularity everything has gone to 28x10.5

What they said. Be careful of the height if going the 10.5 route.

Most I've seen in the past seems to follow this, they still say 29 tall (for now), but it's changing.

https://thenoprepnationals.com/rules/

275 or 28x10.5 as the others have said. NO W's! That's "W" has been a source of contention at a few events...

Before you travel this road and start buying parts. Read the rules very carefully for the class as some mods will not be allowed

So many tracks or org's have different classes based on car build, type of racing event and tires.

There are dudes FLYING now on 235 drag radials, or 26x8.5 slicks.

Shocks are another story......several companies can design/build shocks for class rule specifics and/or personal purpose.

Excellent advice.

January 1 ·
LIMITED OUTLAW:
BODY:
All entries must have a factory based production type vehicle. Must be street appearing, have stock appearing dash. Wheelbase must be within 2” of factory dimension. Factory appearing firewall in stock location. One piece front ends permitted provided it retains stock appearance. Fiberglass/carbon fiber body parts are limited to hood, doors, deck lid, roof skin & bumpers. No fiberglass/composite bodies. Any changes or alterations on body lines or dimensions MUST BE APPROVED prior to modification.
FRONT SUSPENSION:
Back half type cars only. Double frame rails from firewall forward prohibited. NO CHASSIS TYPE CARS. Stock front frame rails and suspension required. Front frame rails to be original OEM type & must be fully intact from the firewall forward out past the far-most lower control arm mount. Notching allowed for header clearance. Header must pass through this area. Aftermarket bolt in sub-frames and k-member allowed. Factory front frame must be connected to sub-frame or k-member or lower suspension by bolting and/or stitch welds...NO solid welding.
Lower control arms MUST be bolted to the factory mounting points on the front section of frame. Any type of spring/shock combo allowed.
Rack and pinion steering allowed
REAR SUSPENSION:
Any type rear suspension allowed
TIRES:
Any slick or pro bracket radial up to 30”x10.5 x sidewall designation allowed. DOT tires, drag radials or pro radials up to 12.5” or 325 sidewall designation allowed.
ENGINE:
Small block N/A
Small block nitrous
Small block turbo
Small block procharger
Small block roots or screw blower
Big block N/A
Big block single stage NOS
TRANSMISSION:
Any type automatic or manual transmission is allowed.
Empire limited outlaw rules

Around here nothing bigger than a 275/15 or a 28 X10.5 non W, a 325 will put you in big tire. Then there is the 235, 26 X 8 class, we don't see many here though.

Same here. I run a 295-65 radial and there are a lot of classes my car doesn't fit b/c of the tire. I'd put a 275-60 on my car if it didn't look ridiculously small. It hooks fine on the smaller tire, just looks stupid to me. Even a lot of the street car classes won't allow anything bigger than a 275 or 28x10.5, which I think makes no sense. Like my 295-65 makes it a pro mod or something?
Occasionally you'll see a class specifically for cars on a 295 or 29.5x10.5 slick, but they are few and far between.
There are SOOO many options for a 275-60 or 28x10.5 tire car that I'd be hard pressed to set up a car for anything else.

Same for my car. I set it up to run 315's. It sits low already. With 28's it look ridiculous. Especially, with it stretched on to a 12" wheel.

For you guys that have gone from these “small” tires to larger or vice versa, how much ET difference between the two?

No real difference on my car b/t the 275-60 (28x10) and the 295-65 (30x10.5).