JeremyGSU

Probably some stupid questions for as long as I have been doing this but I am contemplating going to a 305 squared setup on my C5 Z track car. Are there any downsides? Obviously they are heavier so you probably lose a little speed.

Currently I am using a 285 / 305 combo running Continental GTI slicks on stock rear C5 Z wheels.

I assume going to the wider tire is harder on your brakes? I already feel like my brakes are maxed out (AP T1 kit and stock rears) and I don't know if I want them to feel worse as I am already wanting more. However, I like the idea of being able to rotate the wheels and increase turn in. By going to the wider front does the back end rotate quicker because of less understeer?

I found a huge difference in turn in between my old 275/40/17's and 285/35/18's when I made the change. I can only imagine the 305's make that much more difference.

Any other things to consider?

RDnomorecobra

I went straight from stock c5z sizes to square 18's all around using 305's or 315's depending on what scrubs I can find. It reduces the stock understeer tendencies, which in my experience makes it neutral, not tail happy. Of course finding a good alignment and setup shop helps immensely. You could also run 295's. Only downside may be some rub at full lock or rubbing on the fender depending how much -camber you have.

ZedO6

Subscribed.

Soloontario

One of the issues you might have is rubbing on brake ducts. If you have something like the DRM ones you should be fine but if you have ghetto ones like I do they are constantly getting damaged by the wider tires. OTOH. remember that when the wheel is turned, it may be the height of the tire that does the damage rather than the width.

BTW last year I ran 275/295 Rivals with just neg 1.2 camber and the car was an understeering pig. This year I will be 295s all round and about neg 2.5 camber

Hat_Trick_Hokie

I think you'll like the 305 square setup. I used that all last year and it makes the car very neutral. And being able to move tires around at any corner depending on wear is a huge plus.

I picked up my 305/18 scrub slicks from forum vendor GT Racing Tires. Greg and Todd were very helpful.

Also ran square 295/18 Hoosier R6 with great success.

JeremyGSU

Those are where I got my slicks too. I think I will also like the setup. My other hesitation was the additional strain on the power steering pump, although I can't imagine it will be much. I'm on my 3rd power steering pump since new and 22 track days.

Hat_Trick_Hokie

I mean, you're talking about less than an inch of tire. 285mm to 305mm is a 7% difference. Not sure how much more strain that adds.

For what it's worth, I have over 50 track days running a square setup with the same PS pump. I just turkey-baster the reservoir and refill with synthetic (AmsOil I believe?) every once in a while. Of course now I say that...

waddisme

Hopefully you have already upgraded to the SKF hubs, If not you will be soon. I have run the 305/18 square set up for past 4 yrs. The do rub my DRM ducts some but not much. Also had to upgrade to Turn 1 pump and just did the Turn 1 rack upgrade.

Supercharged111

I only did it on street tires, and didn't really gain any speed going from one street tire to another, but the change in feel and balance made it sooooo worth it. Throttle lift finally produced immediate turn in and entry off the brakes was improved too, at least IMO. As previously mentioned, watch out for those brake ducts at full lock, they will rub! Other than that, you'll be fine.

Last C5

I wondered if the 305/35/18s would rub the DRMs and if the 315/30/18s would rub a little less. Have you tried 315s up front?

JeremyGSU

They would have to. I can get my 285's to rub the DRM's if I go full lock.

Thanks everyone for your input. I will be ordering the 305's today. I'm anxious to see the difference!

ZedO6

Do keep the tread updated after you run the 305's square

braknl8

+1 on the skf's. The smoother you are the easier on the ps pump but be sure you have a cooler that's in good shape. Also, if you are not swapping wheels take note that a 10.5 isn't optimal for 305's. It's not bad but I noticed the additional "squishyness" in the sidewall on stock wheels. Lots of people run 315's on stock 10.5 with great success and I run 305 on them so I'm not saying don't or it won't work...just less than optimal.

JeremyGSU

I have SKF's on all 4 hubs. I've been running 305's on the stock rears on the rear for years without any issues, but I have never run them on the front.

moespeeds

Ive run conti 305s and 315s all around with no issues. They will rub on the drm ducts at full lock but once it wears through just put some duct tape over the hole and keep adding as needed. No issues with power steering, hubs, or brakes. When your ps pump dies get a turn one, and hubs should always be replaced with skf's anyway.

braknl8

...and he saws like a lumberjack.

moespeeds

Lol it took me a week to realize this was aimed at me. I've actually improved quite a bit but when I start to push, I start sawing. It's not nearly so bad in the GTA car!