Size: 285/30R18 (24.8" tall) or 315/30R18 (25.6") for GT3?

[Rad_ Delgado_]

Here is an easy one.

When Ian Stewart (Peter Lier's car) won in 2007, he ran 245/285 A6. Back in the day Ian tested the car with 245/285 and 245/315, and the car worked better on 245/285. In 2008, I think Gary ran 245/285 V710, and he won SS with a GT3 as well.

These days I see 996 GT3s running the new 255 A6 (really a 275/30R18 per specs) and the rear 315/30R18. The 255/315 should bring more grip, despite the taller rear (25.6" in 315 vs. 24.8" in 285), but so far no GT3 has been able to touch the top spots on dry days at National level since Gary won in 2008, no tours, no Pros (except El Toro last year with Randy's GT3 driven by alien-like Braun).

The GT3 is a rear heavy car (about 1,000 lbs per rear corner). It is also a low torque car, putting around 275 ft-lbs to the wheels in the mid range and under 250 ft-lbs at the high RPM range.

So the question is 285 or 315 for National level events?. Assume SS or ASP, given that gearing cannot be changed in these classes.

Thanks for your input.

[Steve Towers]

Well......I think Gary runs Kumho's so I'm not sure about the A6 debate. I believe he alternates between 285 and 305 depending on surface. Probably better if he chimes in.

[Jason Isley BS RX8]

I think Ian ran the 295 rear.

[Gary Thomason]

Pretty sure Ian ran 295's on Peter's car because it behaved better. That's the problem with the 285 - just not forgiving at all of any mistakes, and heats up too fast on the back of such a heavy car. Off course, this is offset somewhat by the gearing advantage of the short tire in slow corners, which the car badly needs. That said, Kumho offers only the 285 and 305, and I tend to run the 285 at El Toro, and the 305 everywhere else. The concrete-like grip at ET allows even an overheated 285 with lots of heat cycles to work OK, making the gearing trade-off a good deal. A hot, worn 285 on a 65 second course in SD is just awful.

The 305 Kumho is almost as tall as the 315 Hoser, so the trade-off is roughly the same when running either brand. Naturally, corner speeds on the particular course that day have a lot to do with the better size to run, with a course having mostly all medium to fast corners favoring the tall tires, and a course with a large amount of really slow corners being the opposite. Pick your poison.

What did you run on the 997?

Did you buy a car? :mrt:

BTW, Rad has probably heard this from me before, so I suspect he is looking for input from people like Marshall and other set up gurus here....

[Rad_ Delgado_]

I just emailed Peter to check on the tires that Ian ran. Let's say they ran 245/295, is it better to run on the heavier and taller 255/315?

I ran 245/285 Kumho V710 on my old GT3. The rear 285 overheated easily, and my tires were shot (no grip) after 4 autocrosses on asphalt.

On my 997 GT3 RS, I ran heavy and tall 19" 265/345 (rear at 26.8" tall), that car never saw the light of competitive National level autocrosses in the 8 months I had it.

Marshall runs 285/30R18 on the Evo instead of 295/30R18, and they could use another 2 mph in 2nd gear, the BSP Evo is heavier and has more torque than the 996 GT3. Toby runs 285 square in his Bimmer Taxi Cab, although that's a pretty light car.

Neither Hoosier or Kumho make a wide tire with low diameter (except the 275/15 for the Miatas).

The 285 and 295 A6 as supposed to be as wide as each other, except the 285 has the short side wall, and the catch me if you can feeling on the rear end (what I remember from my 996 GT3 at a National Tour on them).

p.s. No GT3 here, just intentions to learn something, and probably run a GT3 in a far future

[Jason Isley BS RX8]

I just emailed Peter to check on the tires that Ian ran. Let's say they ran 245/295, is it better to run on the heavier and taller 255/315?

I ran 245/285 Kumho V710 on my old GT3. The rear 285 overheated easily, and my tires were shot (no grip) after 4 autocrosses on asphalt.

On my 997 GT3 RS, I ran heavy and tall 19" 265/345 (rear at 26.8" tall), that car never saw the light of competitive National level autocrosses in the 8 months I had it.

Marshall runs 285/30R18 on the Evo instead of 295/30R18, and they could use another 2 mph in 2nd gear, the BSP Evo is heavier and has more torque than the 996 GT3. Toby runs 285 square in his Bimmer Taxi Cab, although that's a pretty light car.

Neither Hoosier or Kumho make a wide tire with low diameter (except the 275/15 for the Miatas).

The 285 and 295 A6 as supposed to be as wide as each other, except the 285 has the short side wall, and the catch me if you can feeling on the rear end (what I remember from my 996 GT3 at a National Tour on them).

p.s. No GT3 here, just intentions to learn something, and probably run a GT3 in a far future

I have not heard one person say a good thing about the 255. Even cars with lots of camber were killing them. Most people that tried went back to the 245 or up to the 285. Maybe with the light front end they would live on the GT3.

[Gary Thomason]

I just emailed Peter to check on the tires that Ian ran. Let's say they ran 245/295, is it better to run on the heavier and taller 255/315?

Marshall runs 285/30R18 on the Evo instead of 295/30R18, and they could use another 2 mph in 2nd gear, the BSP Evo is heavier and has more torque than the 996 GT3. Toby runs 285 square in his Bimmer Taxi Cab, although that's a pretty light car.

Both Braun and Cris Cox swear the 255/315 combo is the way to go.

The oversteer issues the 285 creates on the GT3 would very likely just result in slightly less push on that crazy EVO - a welcome situation, I imagine. I've certainly never heard any complaints out of the driver(s) of too much power oversteer.

[Rad_ Delgado_]

I got the response from Peter. They ran the 285/30R18, they never tested the 295. Per his words, the 315 made the car numb and it wouldn't rotate. In their back to back test, the 285 was almost 2 secs faster on their test course.

They tested 245/285 and 245/315. Probably the newer 255 (should be called 275/30R18) works well with 315 rear (Braun and Cox), but they are heavier and the rear diameter is taller by 20mm, bad for the SS and ASP GT3. This needs to come down to a same day test of fresh 245/285 and 255/315.

[Jason Rhoades]

This needs to come down to a same day test of fresh 245/285 and 255/315.

Results will only be valid for the course you test on. Not all courses are created equal.
Should also try 255/285. I heard somebody once even ran 285s all around

[Gary Thomason]

I got the response from Peter. They ran the 285/30R18, they never tested the 295. Per his words, the 315 made the car numb and it wouldn't rotate. In their back to back test, the 285 was almost 2 secs faster on their test course.

They tested 245/285 and 245/315. Probably the newer 255 (should be called 275/30R18) works well with 315 rear (Braun and Cox), but they are heavier and the rear diameter is taller by 20mm, bad for the SS and ASP GT3. This needs to come down to a same day test of fresh 245/285 and 255/315.

Plainly, the course Peter tested on must have had a lot of slow corners. Keep in mind my comments about the 285 rears was based on the K-tires - I have never tested the 285 H-tire. However, Cox told me exactly the same thing happened with the 285 Hoosiers. There is no question in my mind that at Heartland Park Topeka the 285 is the only way to go for several reasons (what a bizarre surface that was!).....but on a hot day at Lincoln? Not so clear.

Should also try 255/285. I heard somebody once even ran 285s all around

255/285 would be interesting to try for sure. 285 all around? Now that's just crazy talk...

[Marshall Grice]

BTW, Rad has probably heard this from me before, so I suspect he is looking for input from people like Marshall and other set up gurus here....

I've got nothing. We only run the 285's on the evo cause we can't fit anything taller. I'd love to run bigger if we could.

[Rad_ Delgado_]

I've got nothing. We only run the 285's on the evo cause we can't fit anything taller. I'd love to run bigger if we could.

In BSP you can cut the fenders, the Vette boys do this to their Barbie cars. So, you could run 295/30 or 315/30 or 335/30R18.

[Marshall Grice]

I've got nothing. We only run the 285's on the evo cause we can't fit anything taller. I'd love to run bigger if we could.

In BSP you can cut the fenders, the Vette boys do this to their Barbie cars. So, you could run 295/30 or 315/30 or 335/30R18.

it's not the fenders we have trouble with or they would already be cut. Can't cut inboard of the hub face in SP, which is where our limiting protrusion is. Either way, the point is i'd have bigger tires on the evo if we could. At the same time though i am against large differences in tire size between axles. 245/315 sounds horrible to me.

[Gary Thomason]

245/315 sounds horrible to me.

Keep in mind the weight bias is 64 percent rear. BTW, the stock tires on the 2010 GT3 RS are 245/325, so this is a very normal ratio for this car. The 255/315 combo takes it quite a bit towards less push than stock. Actually, this ratio change just about makes up for the extra push caused by the grip of R-compound tires.

I haven't driven on the 255/315 combination, but have raced on 245/315 - they were clearly better than the combo I usually use :unimpressed:

[Rad_ Delgado_]

I ran 285/335 on my 996 GT3 (under prepared FP car). Boynton runs 285/335 in his monster ASP 600Hp 996 GT2. The 335 is actually the same height as the 315 A6 (25.6").

I ran 265 and 345 A6 on my old 997 GT3 RS. Lots of rear grip. "Junior Johnson" is now running a 345/35R18 instead of the 335/30R18 in the back of the C6 Z06.

Hopefully, a good friend with a 996 GT3 is going to conduct a test with 245/285 and 255/315 and report back.

I drove the Fiat this past weekend with my 2 years old 265x19 from my GT3RS and 40 runs front 295x30x19 in the back, very neutral, turned well, but I lost my rear stability and ability to floor it after apex. It was fun but sloooow.

I should play A-stock to see if the class wakes up.

[Marshall Grice]

245/315 sounds horrible to me.

Keep in mind the weight bias is 64 percent rear. BTW, the stock tires on the 2010 GT3 RS are 245/325, so this is a very normal ratio for this car. The 255/315 combo takes it quite a bit towards less push than stock. Actually, this ratio change just about makes up for the extra push caused by the grip of R-compound tires.

I haven't driven on the 255/315 combination, but have raced on 245/315 - they were clearly better than the combo I usually use :unimpressed:

Well sure, but given that stock cars are designed to understeer (maybe the gt3RS doesn't, i don't really know) do you really want to be anywhere near those tire sizes? With less front tire you need more rear roll stiffness which is not good for putting down power in a RWD car and at some point will undo the advantage of the bigger rear tires on corner exit. If we're talking stock class where you can't change spring rates then you're limited in what size tires you can run, but in SP where you can pick any rate you want i'd be running as large a front tire as possible and a lot of front roll stiffness. Kind of the opposite of an ST car, any load left on the front inside tire is load that could have been on the rear making you accelerate harder.

245/315 sounds much more like a road race tire size where you spend much more time accelerating in a straight line and you're actually concerned with things like drag.

[Marshall Grice]

I drove the Fiat this past weekend with my 2 years old 265x19 from my GT3RS and 40 runs front 295x30x19 in the back, very neutral, turned well, but I lost my rear stability and ability to floor it after apex. It was fun but sloooow.

erase those memories! 2 year old tires are evil no matter what size. Don't make any decisions based on how the car drove with old crappy tires on it unless you plan to always drive on old crappy tires.

[Kristoffer Gjevre]

...
"Junior Johnson" is now running a 345/35R18 instead of the 335/30R18 in the back of the C6 Z06.
...

I have not seen many people run the C6 Z06, so how can I get in contact with this person for some tire advice in case I decide to get R tires for my car?

[Aaron Goldsmith]

...
"Junior Johnson" is now running a 345/35R18 instead of the 335/30R18 in the back of the C6 Z06.
...

I have not seen many people run the C6 Z06, so how can I get in contact with this person for some tire advice in case I decide to get R tires for my car?

Take an Evo school, haha. He seems to instruct at them all.

[Kristoffer Gjevre]

...
"Junior Johnson" is now running a 345/35R18 instead of the 335/30R18 in the back of the C6 Z06.
...

I have not seen many people run the C6 Z06, so how can I get in contact with this person for some tire advice in case I decide to get R tires for my car?

Take an Evo school, haha. He seems to instruct at them all.

No doubt I would greatly benefit from the Evo school, and I need to do that some time in the future...
Hm, stock the C6 Z06 has 19 in the back so I guess "Junior Johnson" is running ASP like the Lynchs...
I was thinking to keep the stock wheels size to have the option to run SS...

Anyway, sorry for the hijack.

[Jason Rhoades]

No doubt I would greatly benefit from the Evo school, and I need to do that some time in the future...
Hm, stock the C6 Z06 has 19 in the back so I guess "Junior Johnson" is running ASP like the Lynchs...
I was thinking to keep the stock wheels size to have the option to run SS...

Anyway, sorry for the hijack.

Yes, you have to keep stock wheel size in Stock, but there are tires available to fit your car.
Kumho makes a 335-19 for the rear, specifically for the C6 Z06. You should also be able to fit the 345-19 A6 in the back. Not sure on your front wheel size but a 315 tire should give decent balance.

[Kristoffer Gjevre]

No doubt I would greatly benefit from the Evo school, and I need to do that some time in the future...
Hm, stock the C6 Z06 has 19 in the back so I guess "Junior Johnson" is running ASP like the Lynchs...
I was thinking to keep the stock wheels size to have the option to run SS...

Anyway, sorry for the hijack.

Yes, you have to keep stock wheel size in Stock, but there are tires available to fit your car.
Kumho makes a 335-19 for the rear, specifically for the C6 Z06. You should also be able to fit the 345-19 A6 in the back. Not sure on your front wheel size but a 315 tire should give decent balance.

Stock is 275/35-18 (25.6") front and 325/30-19 (26.7") rear.

So sounds like I should consider Kumho Ecsta V710 315/35R18 (26.4") front and 335/30R19 (26.9") rear.
The 305/30R18 (25.5") for front is closer in diameter to the stock size and I read something about maintaining the diameter ratio front to back to avoid TC and AH issues, but I turn that completely off anyway so not sure if it matters.

And the other option is Hoosier A6 315/30ZR18 (25.6") front and 345/30ZR19 (26.8") rear.

Thanks, something to chew on...

[Rad_ Delgado_]

And the other option is Hoosier A6 315/30ZR18 (25.6") front and 345/30ZR19 (26.8") rear.

Thanks, something to chew on...

The Vette has massive torque, and my experience with A6 and V710 is that the A6 has more grip on acceleration.

You can either run 315/30R18 and 345/30R19, or 315/30R18 and 315/30R19, or 315/30R18 and 325/30R19 all in A6.

I had every single size A6 in 19" and measured them, here are some facts:

- The 235 and 265 x 19 are as wide as each other, they should be called 245/30 and 245/35 x 19.

- The 315/30 and 325/30 are the same width, the the 325 is taller by 15mm.

- The 345 x19 is a full 1" wider than a 335/30R18.

[Gary Thomason]

Well sure, but given that stock cars are designed to understeer (maybe the gt3RS doesn't, i don't really know) do you really want to be anywhere near those tire sizes? With less front tire you need more rear roll stiffness which is not good for putting down power in a RWD car and at some point will undo the advantage of the bigger rear tires on corner exit. If we're talking stock class where you can't change spring rates then you're limited in what size tires you can run, but in SP where you can pick any rate you want i'd be running as large a front tire as possible and a lot of front roll stiffness. Kind of the opposite of an ST car, any load left on the front inside tire is load that could have been on the rear making you accelerate harder.

Because of the huge rear weight bias, putting power down is never a problem even with the rear bar at full hard - unless (ironically) the too small 285 rear tires are overheated. This happens within about 35 seconds on a normal Socal day on a 285. The 305 pretty much solves this issue. I just wish it was shorter!

In SP, I would definitely run the largest tire and wheel that will fit under the front fender, and like the Evo, 285 is really the limit there.

[Kristoffer Gjevre]

...
The 305 pretty much solves this issue. I just wish it was shorter!
...

So what is considered the ideal max speed in 2nd?