Question on 275's
 

So I plan to put my car on 275 width rubber to take max advantage of the 0.4 favorable power-to-weight adjustment under the new NASA TT3.

I have just been planning to go to 18's and move to the 275/35/18 size tires (Hoosiers).

But what about 17's with 275/40/17 rubber? What are the pros and cons both ways?

What really interests me is the new 275-35-17! I think that might be a great size to try.

And here is why
Smaller wheels improve acceleration at the expense of top end, and taller wheels decrease acceleration while theoretically increasing top speed (though in practice top speed is limited by power and drag long before tire size. Increasing tire size usually also hurts top end acceleration and top speed as it decreases the engine’s ability to overcome the air drag holding the car back)

And not to mention the weight you can save running 17's.

Whats the OD on a 275/35/17, is it published yet?

Are we talkin A6's or R6's?

I think I remember hearing that the OD should be somewhere around the mid-24's. Similar to a 245/40/17 or 285/30/18.

Gotcha. Pesonally for track use I would rather have a larger OD than a smaller. Most of the tracks I run only use 4th and 5th gear. I guess it really depends on the type of circuit that you'll be driving on and how much power the car makes.

Based on Tirerack specs:

Hoosier 275.35.17 A6 - Overall diameter is 24.7

Hoosier 285.30.18 A6 - Overall diameter is 24.9

Well, for TT3, I'm going to end up around 360 awhp depending on exact car weight, but that will be close.

As far as tracks, with NASA-SE, you have Road Atl, mostly 4th-5th with 3rd gear at the T2-3-4 transition, T7 and T10A-B (and possibly T5).

CMP, all kinds of 3rd gear action.

Prob run Gateway and Memphis with NASA Mid-South (and hopefully Barber), lots of 3rd gear stuff at Memphis and also some at Gateway.

Nothing's set in stone yet on the random "penalties" tires like the BFG R1 and S seem to be getting. So if say Hoosier dropped out next year and BFG had TT contingency but the "275 18"" bfg now gets a +0, then the 255 17" bfg tire would be the way to go because it's 10.3 inches like the 255 hoosier was :rolleyes:, or the 275 17 which is almost 11", or the 265 18" R1 which has no penalty yet. Very confusing how they come up with all this.

Regardless I'm running a 275 hoho for the first event with several HC's on them to feel it all out in the new ST3 class and see what the tire vendors do. One thing to note is that the 18" tire will lend you more "time" because it won't wear out or overheat as fast as a 17" because there's simply more rubber to go round.

Yeah, I have looked at the 255/40/17 BFG as something I could run on my current 17" wheels as a stopgap until I decide which way to go and have the funds in line to do so.

Hadn't thought of the possibility of the 18's wearing better.

And yeah, still waiting on the TT3 rules to make sure there are no more 'surprises' vs the ST3 rules ala the BFG 275 deal.

Part of this can be layed on the tire mfg's
275 Hankook C71 - Narrow
275 Hoosier A6 - Medium
275 BFG R1S - Wide

how would 18" tires last longer than 17" if they have the same outside diameter, like 275/30/18 vs 275/35/17

one thing to note is that larger wheels means shorter side wall means less side wall roll.

They don't sell a 275-30-18 just 275-35-18. It's a 35 series sidewall dimension on both the 17 and 18. And, the 17 has a larger width then the 18.

The 30 series tire is the best choice I agree. Just doesn't come without a 285 width.

Forgive me for my ignorance, but is there any problem running a 285 over a 275? Is it a huge penalty based on the NASA ruleset? I'm just a dumb autocrosser :D

^ non stock class like TT3, TT2, TT1 have a power/weight penalty of .4 if tires are larger than 275.

Actually

245 and under DOT Slick, 0.8 favorable adjustment to power to weight
255-275 DOT Slick (Except the BFG R1) 0.4 favorable power to weight adjustment
Above 275 DOT slick, 0 Adjuster on power to weight

TT3 is 9.0 to 1 Power to weight.

Adjusters:

AWD -0.5 Adjuster
4-door sedan +0.4 adjuster
275 Hoosiers +0.4 adjuster

So applying these to the 9.0-1 class target you get an adjusted power to weight of 8.7-1. For a 3160 track weight evo, that means around 363 awhp.

For 285's you loose the 0.4 favorable which makes the adjusted power to weight 9.1-1, or around 347 awhp.

Of course you could run 245's and have an adjusted PTW ratio of 8.3-1, 380 awhp. But I've been on 245's for a while now and I don't think it's enough tire for our cars on the track.

And on the BFG R1 275's, they run a good bit wider than Hoosier 275's, so NASA has declared them in essence a 285 tire, so you loose the 0.4 favorable adjuster.