The Mustang Forum for Track & Racing Enthusiasts

Taking your Mustang to an open track/HPDE event for the first time? Do you race competitively? This forum is for you! Log in to remove most ads.

  • Welcome to the Ford Mustang forum built for owners of the Mustang GT350, BOSS 302, GT500, and all other S550, S197, SN95, Fox Body and older Mustangs set up for open track days, road racing, and/or autocross. Join our forum, interact with others, share your build, and help us strengthen this community!

315 vs 305 Square Setups

9
4
Exp. Type
HPDE
Exp. Level
10-20 Years
Danville, CA 94526
In the process of migrating to a square rotatable setup on my new Apex forged 19x11 wheels. Seems like the go-to is the 305/30. Have been leaning towards the Sport Cup 2 (P0) for durability but never liked how my stock Cup 2s had looked undersized in 305 on the rears as the tread width appeared narrow. I have been close to going with the Goodyear Supercar 3Rs except the nearest I can tell it's 2 track days and they are done - which gets expensive. Got me thinking about why more aren't doing 315 square setups. The tread width/section widths don't look that much bigger, and in some cases are more narrow than some 305s (e.g., R888Rs).

Am I foolish to be jumping past the 305 square to a 315 square setup?
 
I ran the 315/705-19s last year. I didn't like how tall they are for the GT350. They rub slightly on the inner fender cover when turning, and feel the car is just happier with a shorter tire. Unfortunately for the cheap used tires they are the only ones available out there anymore. Was hoping to try out the 305/690-19s this year but cannot find them. I ran Toyo Proxes RR 295/30-19s same weekend as the used 315 Pirelli's and it was a whole different car. Even thought they were brand new compared to the used Pirelli's, the car was much happier coming off of the corners and even in the corners, I ran quicker more consistent times. If I was running more power than stock the 315 Pirelli's may be a good tire, but I feel with stock power the car felt sluggish. I will probably still run the Pirelli's as I have a hard time purchasing brand new tires for fun laps since I don't run in a competition, but just for fun.
 

PaddyPrix

If breakin' parts is cool, consider me Miles Davis
587
825
Exp. Type
Time Attack
Exp. Level
Under 3 Years
San Diego
Are you foolish for jumping past 305? Not really, if you can get a comparable tire in that size, I doubt you're at the point (and who is?) where you can tell the difference in the handling, performance, and track times of a whole 10mm.

I think the major thing is just getting a tire that you can run that meets your budget and performance goals. My whole reasoning behind going with 18x11 rotatable over 19x11 was that there were plenty of Michelin s8m/s9m and Pirelli DH scrubs usually around 400ish a set, which make good HPDE/trackday tires, and then keeping another set for the real events. I really missed out on the 19x11 Goodyear SC3r which are absolute killers for most time attack/trial groups, and if you're military or have a hookup, I hear the USAA discount on Goodyear makes the SC3r a no-brainer, when you're paying about $2k a set of comparable Hoosiers or Yokos.
 
How’s the height of the 315/705 compared to a 315/30?
Here is the link to the Pirelli slicks, 27.75

Here is a link to the Hoosier tires as they have a 315/30, 26.1
 
I’m running 315/30 falken rt660 on 11.5 apex wheels square. Fits that wheel well, and is around .5-1sec faster a minute than the 305 Goodyear sc3 I ran last season. Time trial golden lap is 1-2, but they are still drivable and fun after they start getting warm.

305 sc3 for me wore like iron. Shaved a set to 5/32 for TT nats last year and ran several days after through feb and they were still great and barely worn . the rt660 is burning down faster, but have still had 3 events on them, with pb’s af each one.

This is on gt350, bmr springs and bars, not much camber, around 2.8 all around.
 
Last edited:
"Was hoping to try out the 305/690-19s this year but cannot find them." - used, you won't find them. No race series in north America is on that tire, so really hard to find the 305's in take-offs. Cup Challenge cars ( Ferrari, Lambo) run the 315's on the back, for another 2 years by contract, I think, then tire sponsor gets re-negotiated.

"How’s the height of the 315/705 compared to a 315/30? " Its taller, really f***ing tall, I think more like a 35 or 40 sidewall. I have occasional rear fender rub with the 315's ( 11 square, nonstock suspension, -2.2 rear camber ), but my, do they hook up for the first few sessions..

Then again, I have still never run new slicks, they are just too expensive for me for HPDE. I am 5-6 seconds faster at home track (Thunderhill 3-mile course) on Pirelli DH 315/705-19 than Nitto NT01 305's, and I am paying 120$ per tire instead of 400$ for the Nitto's (or 700$ for a new Pirelli).
 

APEXRaceParts

Race Proven, Street Approved
Supporting Vendor
638
936
Exp. Type
Time Attack
Exp. Level
5-10 Years
Bay Area, CA
I’m running 315/30 falken rt660 on 11.5 apex wheels square. Fits that wheel well, and is around .5-1sec faster a minute than the 305 Goodyear sc3 I ran last season. Time trial golden lap is 1-2, but they are still drivable and fun after they start getting warm.

305 sc3 for me wore like iron. Shaved a set to 5/32 for TT nats last year and ran several days after through feb and they were still great and barely worn . the rt660 is burning down faster, but have still had 3 events on them, with pb’s af each one.

This is on gt350, bmr springs and bars, not much camber, around 2.8 all around.
What size spacer are you running up front with this setup?
 

Bill Pemberton

0ld Ford Automotive Racing Terror
6,530
5,480
Exp. Type
W2W Racing
Exp. Level
20+ Years
Blair, Nebraska
315/30/18 Hoosier R7s with 25mm spacer up front and they work just fine with 2.7 degrees negative camber up front and 1.5 in the rear. So far the wear is good, the turn in is crisp and I have not felt the need to really try to push it to -3 degrees up front as many suggest. I would go with a variety of 315s square without much concern as R7s are quite wide, and even though these are on my Mach 1, even a GT350 is heavy and in my estimation the more rubber on the ground for a heavy Stang is beneficial.
 
63
89
Exp. Type
HPDE
Exp. Level
Under 3 Years
Nashville TN
I've run 325/30/R19 SC3s (non-R) on 11.5" wheels all around for a couple of track weekends and have been quite happy with them. No rubbing, good even wear, good grip. 35mm spacers up front on OPR GT4 studs, 3mm spacer rear out back for peace of mind on OPR mid-length studs. All 4 wheels are ET56 (Apex). BMR springs, -3* front, -2* rear.
 
196
208
Exp. Type
HPDE
Exp. Level
20+ Years
NC
I've been wondering the same thing. If a 325 on an 11 or 11.5 wide wheel would fit under the fender of a regular S550. I'm shopping for wheels and tires due to finding a cracked wheel and a bad tire repair. 305 Goodyear's are on perpetual backorder; although the R's are available
 

Bill Pemberton

0ld Ford Automotive Racing Terror
6,530
5,480
Exp. Type
W2W Racing
Exp. Level
20+ Years
Blair, Nebraska
I think the Rs could be a very good choice, as they seem to keep up with quite a few of the fast rubber. It seems they wear well on smooth tracks , though one often hears about fast wear by some , but I wonder if they rotate the tyres enough?

I have two weekends on a set of Hoosier R7s, both at Heartland Motorsports Park, and I am quite confident I could easily get two more there, and still have rubber left for Motorsport Park Hastings ( which is also super smooth ). Tire wear has many factors that inhibit wear and track surface is often right up there at the top.
 

TMO Supporting Vendors

Latest posts

Buy TMO Apparel

Buy TMO Apparel
Top