You seem to be an arrogant prick lol. He already stated in the op it is for a particular class that is based on tire size.

The OP then stated AutoX. SCCA has two rules when it comes to tires. One of the rules is you cannot modify wheel wells for street class cars. Obviously they is in street class with that car. They are not running a 275 or larger on a Miata without modifying the wheel wells It sounds like the OP is talking out his/her ass to sound like they know more than they do. I'm paid for a living to make cars faster, I'm well aware of all the various forms of racing and what goes along with them.

op said like autox, so im guessing hill climbs. Class ima guess with that amount of hp would be sm

I'm going more or less the OP likes cars, wants to pretend they know more than they do and just didn't know enough about what they were talking about. Which is why I tried to give them advice, like stop worrying about putting retardedly large tire size on a car that doesn't need it.

for autox 275s are pretty common on miatas

Check any autox or Miata forum. 95% will tell you don't go bigger than 225. MAYBE 235. It's unnecessary and not to mention illegal on a street class car (which is what the OP car is). You cannot modify the fender to fit wider tires, which you must. Just because other people do it, doesn't mean it's right or not. Unsprung weight is very important in track cars and having that much more wider of a tire isn't ideal when it's not necessary.

Have you not realised ive been talking about her future power goals?

400hp is nothing, auto-x and 275s and bigger is overkill. Especially in such a balanced car as a Miata. Again, unless she's going to race against non-street class cars (which shes not), any tire size probably above 245 won't work for her. Check out Shandelle's builds. They build full v8 track cars out of Miata's. You're talking 400 horsepower and more and all their cars run 225. They did a amazing lap at VIR with only 225s. But somehow that person is talking about like 385s? Or whatever ludicrous number they came up with? Again, that person has a lot to learn and I was just trying to help. They going to look like a fool out there on some wide ass tire that they probably couldn't even really fit, all for 250-400hp.

seems like he races in GTA. SCCA and NASA will not put a miata with 400hp in any street class except ssm

I think they want to get into racing. Probably race on back roads like stated and wants to pretend to know more. Without being a dick that's just what I think.