brel

How much difference in diameter between front and rear tires can 718 GTS (2018) tolerate before all electronics get confused on a track? I have a set of RE71Rs 255/35/19 (26" OD) and 305/30/19 (26.2" OD) on OZ’s for my 981 GT4 and I'd love to use the same set of track tires on both cars

Todd B

Those would be fine.

Haros

Mounting diameter is not the same as tire diameter - and tire diameter, as well as, actual width, vary manufacturer to manufacture.
The acceptable tolerance is generally 1% or less.

So with that in mind, you are on the cusp of 1% with the numbers you gave, and I wouldn’t be surprised if you mount it would be even closer.

Your car is also pushed with the back wheels, so it really doesn’t matter in actual practice as the fronts just follow the rotational speed of the rears.

Z06jerry

Haros, What is source for that info? Even stock GTS sizes differ by by 3%.

Haros

https://www.1010tires.com/Tools/Tire-Size-Calculator

you can see tire calculators themselves state “tire sizes that are with in plus or minus 3% variance”

1% being optimum.

I have a rolling resistance guide in one of my old engineer books I can take a picture of you like when I get back into town. It’s essentially boring math.

If you look at manufacturers websites, you can see the exact same tire profile has an overall different circumference tire to tire, brand to brand.

it ultimately doesn’t matter unless you are throwing off a speed sensors, which modern cars use in each wheel. This would be an extreme situation though.
You have to remember that each tire will ultimately suffer unequal wear and the ratios change minutely.

My Michelin tires are nearly less than 1% in difference and my NITTOs being 4% difference front and rear of the same F/R bias as an example.

brel

Thanks for the input. FWIW, there are no issues during "normal" street driving. Of course, none of the "nannies" were ever activated. I believe they all based on rotational speed variations of individual wheels. I assume, Porsche designed it to use relative speeds and not absolute values and built-in enough tolerance to compensate for tire wear and manufacturing differences. I guess I'll find out next time I'll get the car on a track.
I also learned that 981 TPMS sensors are not compatible with 718. Bummer.

Z06jerry

Haros, you have a GTS 4.0 on order right? The tire size calculator you reference confirms that the Cayman GTS stock sizes 235/35-20 & 265/35-20 vary by 3%! Your 1% maximum F/R deviation claim is not reality for a Porsche GTS 4.0.

donR

Is there a 265/35/19 or 245/40/19 available for the front and a 305/35/19 for the back in anything? These sizes would be near perfect.

Seems to me 19" track tires for these cars almost always end up being smaller diameter due to the restriction in sizes. Really hard to find perfect fitment. Maybe a slightly smaller diameter tire is better for the track?

brel

RE71R's and GOODYEAR SC3 are available in these sizes.

Not sure why it's "perfect". For 718's to maintain the factory setup, the rears should be .8" taller than fronts.

Yes, smaller OD gives you a bit more torque and lower CG.

Z06jerry

Fitting 19" (or 18") rear wheels wide enough for a 305 could be a challenge on a 718 due to interference with the toe control link. 20"s are large enough to provide clearance.


Agreed, although the GT4's rears are only .2" taller than the fronts. For myself I will choose tires that fall within this range , .2" - .8".

sobiloff

APEX figured it out for 19" wheels with their Front 19x9" ET50, Rear 19x11" ET46 SM-10s; see https://support.apexraceparts.com/hc...-Fitment-Guide

ETA: Here are some additional details on rear toe link clearance from their recent gangup:
"Factory Toe Link Clearance With 11” Rear Wheels

Since the GT4 first debuted, there has been a lot of chatter about rear toe link clearance with 19” wheels. Slimmer 10” and 10.5” widths are non threatening, however, 11” wide wheels leverage all of the available real estate under the rear fender wells, so it is imperative for the wheel to have the right offset (aka backspacing). Too high of an offset and you have inner barrel contact with the factory toe link. Too low and you are all over the fenders.

Through 3D laser scanning and on-car fitment testing we optimized the position of the 11” wheel, resulting in a clean fitment allowing drivers to keep their factory toe links."

Z06jerry

^^^^ good to know, these will be my choice.

brel

OZ's work as well. I've been using OZ Legerra HLT 19"x11 ET50 with 5mm Porsche spacers (45mm effective offset) on my GT4 for over 4 years, I've tried them on my 718 CGTS and they seem to fit just fine, as you can see on the photos.

donR

I thought Porsche had set up our cars perfectly.

That's interesting, what is the rational for having rear wheels that are slightly taller than the front ~ 0.8"?

Also I notice most track wheels with offsets increasing the front track relative to the rear making more a square set up (e.g., Apex over OEM is +24mm wider on the front vs +9mm on the rear). Similarly I noticed the new 992 Cup Car has gone very wide on the front track, I think also square set up.

Whereas on the OEM 718 setup is 24mm narrower track on the front so a little staggered, 1515 front vs 1529 rear.

Alot of these aftermarket track wheels seem to produce slightly wider track at the front. Maybe like the Cup car?

I now notice the GT4 also has near square set up, front track 1538 vs rear 1534. So I assume using these track wheels on a GT4 makes the front track again much wider than the rear.

I assume what is better for the track is maybe not better for the road?

Z06jerry

I'm not concerned about their rational for doing it, I just want to make sure I stay within their parameters for ABS, Traction Control, PTV etc.

IMHO for offsets, the only thing that matters is suspension clearance, bodywork clearance, and the rules governing the competition. For example GT4's have a narrower track than GTS's because they have wider tires to fit under the same bodywork.