1. NASA classing favors tires labeled as 245mm or narrower, as mentioned above.

2. 245's are easier to fit under massaged Miata fenders than 275's

 

For me Im on the ragged edge of the wt/hp to run TT2. ~16 hp gain to run 245 and under and it puts me at a hp level I can achieve with just turning down the boost rather than making the car run bad or adding lots of ballast.

Which is faster assuming ~ 16 hp lost while running fatter I don’t know for sure? I set some pretty fast times on 225 Nitto’s though in the past and the 245’s look like they might have the similar sweet spot for sidewall stretch that so many people have observed including myself when mounted up on the 10’s more so than the 275’s

 

Ah, thanks, good answers.

I guess I was thinking of my friend ZX-Tex and his LS1 Miata and his quest for grip with a heavy car.

 

This.

I always knew the 275 really wanted a 10.5 or maybe even 11 but clearance would become even uglier with that width. On paper, the correct size for the 245 is a 10". So yeah, the 245 on 10's should be a marriage made in heaven, just the right amount of sidewall tension.

 

Does (2) mean they're likely to fit without cutting/flaring the fenders?

I'm currently looking at upgrading from my 225 A6s (for autox, so (1) is irrelevant to me), and had basically resigned myself to having to uglify the car by cutting the fenders and adding plastic flares to fit 275s. If 245s will fit w/o uglification, that's got real value.

--Ian

 

Conjecture on my part. Folks have coerced 275's on 9's under ravaged but un cut NA fenders. Maybe Bob will post pics. He has flares though.

 

Speak of the Devil... I just stumbled into this thread. So yeah the 275 A6 grip levels are magical, and i even managed 3 fast laps at NOLA before they overheated. The additional corner exit grip is also very handy.

But, I am definitely interested in the 245 since, as mentioned above, by the NASA rules I can add power back to the car versus a 275 since the correction factor is different for a 245 vs a 275. In my case it amounts to about 13 HP for TT3. I have been wanting this option, and now that it is available, I may have to try it since only testing will tell for sure. It might be a faster setup at the high speed front-straight tracks like TWS and NOLA (140+ mph).

Thanks for the pictures Bob.

 

This is the other factor. Even with substantial power adders, the Miata is sensitive to tire scrub and aero drag. No precise science in my estimate but I think most full sized road courses will favor the 245/40 up to about 260whp or so. Past that power level there is enough torque to maintain optimum slip angle and actually make use of all the surface area on a 275.

 

Educate me how I get these in Holland, and how I fit 10s on an NA body?

 

For me to get the 275's to work with the et25 offset 15X10 6UL’s I needed 5mm spacers to keep the tires from destroying themselves on the shock tower area of the front wheel well and the inboard vertical wall area of the rear wheel well. I don’t know how people are getting these to work with less offset like on the et36 9” wheels with no spacers it just doesn’t seem possible to me unless your suspension setup has crummy bump travel and very little camber. Thus the fender lips are majorly in the way on the outboard edge.

With the 245’s I can take the spacers out and I suspect you can likely get the outboard lip to work without cutting but it would likely take a flat roll and a reasonably doable amount of pulling.

Suspension setup will have an effect. Bump travel and static camber settings. NB’s also have more room by like 5mm.

 

Ok those worked better than anticipated. 245/40/15 R6 on 10”

NASA Race Weekend set TT2 lap record by a whole lot at Pacific Raceway given that there is not a lot of Nasa history on the track. 1:27.457 is like GT2 car lap record territory however and Im doing it in a street driven Miata. ~13 seconds under spec Miata lap record. I was 4 seconds faster than what I got on 225 Nittos before and was running lower boost to fit in the class while doing it. Sidewall streatch is more optimized than the 275's I think. they are less squirmy feeling and the brake away is more linear. seems like the 275's want 11" rims.

Dynoed at 324.6 hp and weighd 2505 with driver.

 

I'm sold.

You had me at "4 seconds faster" .

 

My closest competition was a 370 hp S2000 running 245 Hoosier A6. It is questionable whether I had area under the curve advantage on power though. I have a mountain of torque in the usable rev range where he makes peak power at redline and 370 hp on the tip of his power curve means he had to be a lot heavier.

 

What it looks like on shaky video going ~4.5 seconds faster than any Miata based car has gone before at Pacific Raceway. I started doing turn 2 in fifth about 100mph at the apex and had the tires squealing as I was trail breaking it in from 150mph down the straight.

Fastlaps at Pacific Raceway NASA TT2 lap record in Turbo Miata 9/14/2013 - YouTube

 

Congrats Bob! Thanks for the good data!

 

+1 Good job Bob. Thanks for the info.

 

Last couple weekends Set Miata lap record at The Ridge Motorsports Park 1:48.742 at SCCA time trial where the Spec Miata lap record is 1:57.983 . And won NASA TT2 again with a Miata lap record at Portland International Raceway with a time of 1:21.412 where the spec Miata lap record is 1:30.448.

Had some more competition at Portland from a couple fully built BMW M3 Race cars running Yokahama super soft Slicks. One E46 and an E92 As well as the 370hp super charged S2000 also running 245 DOT Hoosiers A6 though. Only had 8 tenths on the tight group behind me all within a tenth of each other after breaking my 6 speed at the beginning of the final session of the weekend and everybody else setting personal all time bests that session.

Car is working pretty good on 245/40/15 R6 mounted on 10” wheels. Also found Pacific Raceway is a track I master better than most.

 

 

Warn us with a NSFW tag, jerk.

 

The purple crack!

I did pretty good this year. won a set on 275's autocrossing SCCA NT's which I used it to get 245's and with those 245's I won two more sets of tires. got another set of 275's and another set of 245's. Thanks to NASA TT in their first season of running Northwest tracks.