I mounted a 10" lr tire on a 8" wheel just messing around with old stuff. Looks pretty goofy so the question stands, does anyone actually race like this on dirt? I've always ran a 10" rim. Thanks

Nothing unusual about this, if you look at the spec sheet for the tire, that is OK, that is a way to chg stagger also. You just accidentally learned something! Bob

Best way to learn.....lol

Lots of cars run 10"RR 8"LR wheels on the midgets. Not sure about the minisprints, I think they run wider rr. As bob stated you can do a lot with some different wheels as far as stagger goes. We used to have 2 lefts and two rights of different widths to make it easier to get more or less stagger without inflating the hell out of the tires

Same here. Makes dialing it in a little easier with more rear choices.

IMRA runs rules very similar to the Montpelier Midget Series, with a few exceptions; one of the exceptions is a rule limiting left rear wheels on both Chain and Shaft-driven cars to 8", but with a maximum tire tread width of 10" (MMS has no LR rule). Right rear rules are similar to ours; 10" max wheel width for Shafts, 12" max for Chains, 12" tread for both.
Not sure why you'd run a 10" tire on an 8" rim if you didn't have to, other than for more stagger choices as has already been discussed; how much more rollout do you get on the narrower rim?