bowmanm98

I am trying to decide which tires to get. I can either get the 275/35 or the 305/35 on a 18x10 inch wheel. I'm definitely getting the 245/40 for an 18x8 on the front. Which will be faster? Toyo RA-1

JimB

Normally I would suggest the 305s but the 35s are very tall. .7 inches taller than the very tall Hoosier 315/30s I run. That's pretty tall gearing for your car. On the other hand, 275s are pretty narrow. Tough call. You might want to consider a Michelin PSC. They have better sizes for your car.

speedread

I've had very good luck with my set up of 225/40/18 and 285/30/18 with rolled fenders. I don't race, but run in the middle of the top group at DE.

bruinbro

Why is the 245 a 40 aspect ratio? Aren't they usually 35?

Bro

10 GT3

I would not recommend Toyo RA-1's for this application due to the limited sizes. For 17"s they have the sizes that fit, but they don't for 18"s. I would suggest going with Michelin Pilot Sport Cups or Pirelli P-Zero Corsas in the original 225/40-18 front and 285/30-18 rear sizes for your wheels. That will get you the best results. For track events they will be grippier than the RA-1's and you will get good life out of them. The MPSC's need at least 1.5 degrees negative camber to work properly and are very tolerant of high heat. The Corsas are less tolerant of heat, but don't need as much camber to work properly. I run 235/40-18's MPSC's in front on 18x9.0" wheels and 295/30-18 MPSC's in back on 11.0" wheels and this setup is extremely effective in making the car neutral with high grip with great turn-in.

speedread

I'm no expert. Kelly Moss Racing set it up for me.

Russ Murphy

Bro's talking about bowmanm98's front tire choice.

Maybe try to find one of the suggested tires in that's offered in a 245/35. That would have the same sidewall height as the 285/30 which would be a good pairing imo.

BluemaxxRacing

JimB - Are you running a 10 inch rear wheel and still running a 315 tire? Just wondering if I would be able to run a 305 tire on my 10 inch rear fikse wheel??

10 GT3

Here are a few note about choosing tires. If you want to learn more, I would suggest tasking the Evolution Driving School Dial-in Course (run by multiple SCCA national champs).

1. If the weight of th car has not changed are you are running the same tire pressure with 2 differnt width tires, which has the larger contact patch?

Neither, they are the same. Only the shape of the contact patch changes. With the narrower tire, the contact patch is more narrow; but longer. The wider tire has a wider, but shorter contact patch.

2. How do you increase contact patch for more grip?

The only way to increase contact patch is to run a lower tire pressure.

3. The larger the tire, the wider the section width. How could this be an issue?

A larger section width can significantly effect turn-in. A stiffer sidewall, will have less flex and react quicker. A wider sidewall will always have slower turn-in. This is because when you turn the wheel, the wheels first change their angle of attack. The car continues to want to go straight until that angle of attack has changed at the tread. Since the tread has friction with the surface, it resists movement. The sidewall flexes until it becomes full stiff and the transiant loads allow the car to turn.

So now think about putting a wider tire on the same wheel, you have to decrease tire pressure to get more contact patch. This makes turn-in far worse than the original tire setup.

4. So what does all this mean?

It means run the rigth tire for your car a fluxuate tire pressures for optimum grip. If you want more grip, you not only need a wider tire; you need a wider wheel to run it on. This reduces sidewall flex and allows lower pressures to be run.

Tou don't have to take my word on this, just try it yourself and learn the lesson. The worst it will cost you is $1000 or so for a set of tires. I wasted an entire Solo 2 season dropping from class leading to middle of the pack due to mistakenly going to wider stires on stock wheels in stock classes. I attended an Evolution Driving School Dial-in course and learned these lessons. My times are far faster from them.

Russ Murphy

Thanks for the insight 02. Your explanation makes perfect sense and has shown me the sometime error of my ways.

bowmanm98

Which tires are faster the Michelin Pilot Sport Cups or the Pirelli PZero Corsa System.

Both have porsche specific sizes. 235/295 My guess is that the Cups are better in the dry and the P System works better in the wet.

The cups are 80 treadwear and P System is rocking the 60 rating.

I'm too lazy to change the tires at the track before and after for the hoosiers.

JimB

I think 305s would be too wide for 10 inch rims. I have 9 inch in front and 11 inch in back.

earlyapex

Don't pay attention to treadware ratings. They do not show how much stick the tires will have on the track. You also cannot infer how the tire will behave over time. That said, you simply cannot go wrong with the MPSC on a 996.

mitch236

I would think your wheel width in the rear is too narrow for the 305. That being said, I run MPSC's on my 996. I have 9" front 11" rear wheel widths (18") and run 235F 295R and this works extremely well. At Sebring, I run 2:29's which compares closely with the Hoosiers I used to run.

bowmanm98

What did you run on the Hoosiers? I'm wavering again and starting to think I'll just go hoosiers since I don't go racing that much. The tires will probably get old before I can use up MPSCs. Doesn't the rubber have a shelf life anyway?