DR911s

This post is largely a repeat of one I tried to post earlier today. I'm a newbie. Apparently I messed up in making that first attempt or a moderator is still holding it for reasons I do not know.

I recently purchased a CPO 2017 911S equipped with Rear Wheel Steering [P3P Sport Package] and from a Porsche dealer which came with Pirelli Sottozero winter tires, 245/35R20 fronts on OEM 8.5x20 wheels and 295/30R20 rears on 11.5x20 OEM wheels with the tire pressures set at 39 psig front and 45 psig rear. [I know that Porsche and Tire Rack state that such 295/30R20 tires should be mounted on 11x20 wheels.) The ride quality was horrific; I thought I was going to destroy myself or the tires on the drive home on the mediocre condition Cleveland, Ohio area roads! The dashboard display for TPMS only displays choices for full load; there is no display of “Comfort Pressures” choices. So I unchecked the “Full Load” pressures and reduced the tire pressures to 32 psig front and 37 psig rear after the car had sat in my garage overnight at an ambient temperature of about 65 degrees F. The ride quality is much better, but now the risk of rim damage is increased.

Also, due to the configuration of my driveway and garage, the front water/air deflectors in front of the front wheels slightly scrape when entering and particularly when exiting my garage.

So... I'm planning to buy another set of wheels with summer tires mounted and balanced and equipped with TPMS sensors. My goal is to have a car I can enjoy driving on extended distance road trips, e.g. from Ohio to Florida; I'm willing to sacrifice a little acceleration and handling/cornering capability to improve ride quality and reduced risk of wheel rim damage. This car is far more capable than my recently sold BMW E36 M3 with modifications!

I'm presently leaning towards purchase of the O.Z. Racing Leggera HLT wheels, 9x20, 50mm offset fronts and 11x20, 65mm offset rears fitted with Michelin Sport P4stires. The recommended and Porsche approved “N” rated tires would be sized 245/35R20 for front positions and 305/30R20 for the rear positions, both (Y) rated and equalling or exceeding the minumum load ratings required for this vehicle.

Can larger OD tires be fitted to these O.Z. wheels or the OEM wheels and used on this vehicle? Specifically, can Michelin PS4 tires in 245/40R20 or 255/40R20 front positions and 295/35R20 or 305/35R20 rear positions be used? Per data published in the “Detailed Specifications” associated with each particular tire at Tire Rack, the differences in OD are about 0.9 inches [e.g. about 3.4 percent larger]. I note that the OEM tires combinations result in an OD differential with the rear tires being about 0.4 inches larger OD than the front tires, and given that the rear tires generally wear at about twice the rate of the front tires [in normal public highways use], that front to rear differential grows as time and miles accrue. I also note that with the Pirelli winter tires fitted, at a nominal speed of 70 MPH on an interstate highway, the actual vehicle speed is about 2 MPH low compared to the speed calculated by various GPS systems. Thus my proposed increased tire sizes would appear to likely result in actual vehicle speed being about 1 percent higher than indicated via the dashboard speedometer.

1) Would these larger tires on these O.Z. wheels cause any fender clearance issues?

2) Would use of larger tires than OEM cause any issues with the electronic stability management system[s]?

mgordon18

Wow. There's a lot to unpack up there.

Your winter tires are properly sized. Save them for November. Your issue was pressure. Put them at 31/34 and go about your business. There won't be any rim damage unless you hit a big pothole.

For summer tires, just go with the manufacturer recommended 245/35/20 and 305/30/20. Don't mess with what works!

DR911s

Thanks for your quick reply.
Do you know anything about how the Porsche stability control system works, and particularly if it would be upset and cause dashboard warning lamps to light up if different sized tires were fitted. Please note that I am very well aware of how aircraft brake control systems operate due to past employment, so I am looking for more than basic information in this regard. I cannot think of a single car that I have owned where I did not eventually use non-OEM specification tires including on a Corvette, several BMWs including the 1993 325i converted to 1995 M3 specifications and beyond, and my 2011 Audi S4. Many compromises are made by OEM, based on many factors. I'm expecting that the majority of those participating in this forum are well above average in knowledge of Porsche vehicles and cars in general.

Bob Z.

Have you searched the forum? I think I read something about this a LONG time ago.

koala

PSM won't factor in to minor sizing changes. Just have to do the math to ensure you know how much your speedo will be out.

Agree with tire pressure being the main factor. I've never owned a car that is this sensitive to tire pressures. I have the OEM Pirelli rubber down to 30/34 and it's finally behaving the way I expect it to. Just need the DSC V3 to try and rid the car of some final annoyances in driving behaviour.

Rennolazine

1. Lower the pressures. Its called comfort pressure for a reason
2. Ps4s >>>> p zero in every way
3. All of the configurations above should work regarding clearance
4. The 295 tires drop the ride height 3mm im the rear (thiner side wall profile). 305 ride better.
5. You might need a dsc v3 controller when its all said and done

Stevelev

If you’re looking for improved ride, definitely drop the tire pressure down another few lbs. I’ve rarely found the ride quality of winter tires as good as summer - on my S3, my 18” winters with more rubber between the road and wheel are no better than the 19” summers. For what it’s worth, my previous 991.2 with RAS, PDCC and SPASM needed 30/ 33 cold pressures to reduce the jarring ride on our less than perfect roads. My Targa with PASM and OEM 20’s provides a far smoother ride with pressures set at 33/ 36. I too have gone to non OEM sizing on other cars and have read that anything under 5% of OEM sizing shouldn’t cause a problem although I prefer to simply stick with OEM recommended sizing.

DR911s

Thanks, Rennolazine, but in my car there is no display of the option "Comfort Pressure." Only "full load' which box I unchecked when I lowered the tire pressures.

I thought I had read somewhere in the Porsche owner's manual that Comfort Pressure wasn't available in every country, and that decision was made by Porsche according to jurisdiction.

I'm going to try pressures that are a little lower than at present. It's the rear positions I am most concerned about. The ride and handling characteristics of the front tire positions for highway driving are fine to me in the 32 - 33 cold [ambient] psig range... they move up to about 35 psig when driving on the highways.

achtung6

Your handle says you are in NE Ohio...as far as I know, US Carreras allow for a "comfort" tire pressure setting. I know my 2017 C2S has it. And as a side note, running the factory recommended pressure on the street ruins the experience in this car. The tires are hard as rocks at those pressures. I run 29F/33R cold. No problems with the OEM Pirelli or the Michelin PS4S fitted 300 miles ago.

GKGEIGER

I'm also in NE Ohio. I recently had my car serviced by a dealer who set my tires at the door sticker settings. As soon as I left I knew what had happened. When the tires got to temperature the fronts read 39# and the rears were 49#. I couldn't wait to get home and lower them to 30 and 33. What part of NE Ohio are you in?

DR911s

GKGEIGER, I'm in Akron.

Homeles

255/35/20 x305/30/20. PSS 32/36. US spec 2013 C2S. No problem

Valvefloat991

But first calibrate your stock speedo. Larger tires will probably make your reading MORE accurate.

GKGEIGER

Akron also, Portage Lakes.

koala

Indeed. Actually, one of my favourite ways to beat a speeding ticket is to produce a certificate showing the speedo. was inaccurate. Zero points/faulty equipment fine is better than the points and insurance increases.