All,

I'm looking to buy a set of 275 A6's for autocross. I had a set of 225 A6's on 15x9's and the tires measure only about 1" narrower than the 275's. Before I invest in flaring or cutting fenders, I wanted to see if anyone in the community has made a jump from 225 to 275 A6's and what kind of improvements they saw in grip, lap times, driving impressions, etc.

Thanks.

 

I know the difference between 255R13 and the 275 man meats is worth the time if you compete at more than a local level. But its not enough time that if I just ran with the local miata club and occasionally the scca that I would cut up the car to fit the 275s.

If you run the 275s on 6ULs you can get away with just running a 5mm spacer up from, and cutting the front fenders. And using a fender roller to pull the **** out of the rears. Although some people report not being able to pull the rears enough but being able to pull the fronts enough. Try both and see if they fit. You wont be able to pull the car far enough to fit the 275s on 15x10s.

IMO its worth it if you are competing regionally or nationally. But if its just local I'd probably go with the 255 13x9 combo and not cut. I dont think anyone is going to have the data of going 225 to 275, maybe Bundy did it a long time ago, or maybe someone who's been in CSP for a long time would know.

 

Been a long time since i've seen jman here but he was running 275 on pulled fenders.

 

Those look like 15x9s. Which arent too bad, but maybe half a tenth slower than the 15x10s. I MIGHT have been able to pull my front fenders more if I had more patience but I started to tear the metal on mine. It was easier to just cut them and put some sheet metal edging on them.

 

The picture is quite old (the front weren't even pulled back then because a simple roll worked fine with the 15x9) - I've been running 275 & 15x10 6UL for two years now. I just pulled the front fenders now and the rear fenders more. Used a roller tool. Fenders slightly deformed so it can't win a car show anymore, but it works for racing

To the original poster - can't quote times but the grip of the 275 is noticeably more than the 225

 

Man I'm not sure how you got the rears out enough. I almost pull the car off the jackstands just trying to do the 15x9s with the eastwood roller.

 

That's some MONSTER rubber, I can only imagine that vs my 205 starspecs...

 

His name was Mike Perry and he did a L33 swap NB. He found that Hoosier 275 were around $300 each and The Official Site for American Racer® Racing Tires 275 were around $160 each with the difference to be around a sec slower than his fastest time.

http://www.v8rspec.com/forum/viewtopic.php?f=10&t=15

 

I've also fit 275s on 15x9's on mine with just a roll & pull, but be aware of your shock/spring OD. The Tein's I have are bigger OD than most and the tire rubs on the springs. I stopped running them until I change the suspension.

 

Perhaps a set of 12" wide midget tires? Needs a 13" Dia rim though...12" is 305s

Micro Sprints Asphalt Racing Tires

 

would a spacer help?

 

MARK Perry
I also tested the American Racer tires. More info here:
New 275 racing tire option !!! - Page 2 - MX-5 Miata Forum

 

FWIW hoosier just released a 245/40/15.

for autocross the 275 will always be faster as long as you're making a little bit over stock power. For road course use the power crossover point seems to be the mid 200's.

 

It would if I had fender flares. With just rolled & pulled fenders there isn't enough room for 275's + a spacer.

 

Thanks for the input. The fitment info sounds promising. I went ahead and ordered a set of used 275 r6's for trial and fitment. The tires aren't much wider than the 225's and the unmounted tire bead width is 9.5" so I'll have to pinch them slightly to fit on 15x9's.

Emilio - the 245 size looks enticing, but I'm surprised to see how similar in dimension it is to the 225. Almost the same. I would have thought Hoosier would have placed the sizing closer to the middle of the 225 and 275.

 

IMO the spacer is required for 6ULs in the front even with the 15x9s on xidas. And I have the built up hoosier on my upper shock mounts to prove it. And the wear on the plastic spring isolator. It also puts a nice purple ring on the hoosier and will cut down to the cord in a couple auto-x's. If you're not doing that you either have less compression travel than I do or you arent cornering very hard. Or I guess you could be running higher than 850 pound springs.

The other question, is there an Avon A11 compound thick kevlar construction tire in ~23" diameter that is this width? Because that will be WAY faster than an A6 if you didnt need the tire to be DOT.

 

The tread width differences between a 225 and a 275 are pretty significant.

This pic is of 225/45/15 Hoosier H2O wets on 15x9's vs 275/35/15's on 15x10's.



That said if you aren't going to run 15x10's then sick with 225's. 9" wheels pinch the sidewall of the 275. You will see significantly more shoulder wear and less overall performance. Keep in mind too that to optimize the 275's vs 225's would at minimum require some shock adjusting and more than likely require a change in spring rate. So even doing a back to back test on the same day(without optimizing both setups) isn't going to provide you any usable data.

This picture show's 275s on 15x10's vs 275's on 15x9's

 

That pic might be a bit misleading. The H20's are significantly narrower than the same size A6/R6.

 

I can't believe this has not gotten more attention. There has been sooooo much interest into that tire size I would have expected a stronger response.

 

It is step 1. Most people are still waiting for a street and/or medium compound more akin to NT01/RA1(cheaper).

Lots of us are going from fast HPDEers that would have loved to try 245s, to TT/PTers that need the soft and skinnies for points. It will be fun to watch Emilio and Andrew try the 245s in the higher classes maybe?