Jeff928S4

1989 S4

Recently got some 18' Turbo Twists off of Garth S's CS.

After some fending rolling (I think Garth has a few pics), we were able to squeeze the 285/30/18's on the rear. The car now looks sooooo much better, and the larger tires give that "offset" look from the rear.

I have also found the handling to be improved.....the car "feels" about 1000lbs lighter.

Now, some questions...

1. Will these wider rear tires effect fuel economy?

2. Going from a 245/45/16 to the 285/30/18......will my speedomoter read wrong? I think that tire gives me the correct ride height, but will it throw the speedo off?

3. Why didn't Porsche use a bigger rear tire originally? Perhaps the 285/30/18 wasn't really around then?

cdbtx

Yes there will be a difference on the speedo - but do you actually look at yours?

http://pw1.netcom.com/~sgalaba/tiresize.htm

The factory phone dial rims are very heavy and I noticed a huge difference in handling when I changed mine.


Chuck

83 Kiln Red 928S

Jim M.

Your speedometer will read 0.207% too slow, or at 100MPH on the 245's the 285's will read 99.79 MPH. Not enough to care about!

Jim Mayzurk
93 GTS 5-spd

Jeff928S4

He he - I don't usually look at the speedo, now that I think about it.

Did the conversion and apparently, if it now reads "100", in reality, it is "99.79". That's pretty close. (oop's - didn't see Jim's post)

I had the dish-style before......are they roughly the same weight as the phone dials?

The handling is wayyy better. My 928 now feels like my first Porsche, which was the 944S. There is a feeling of less "side-to-side" roll from the car when going around corners.....feels like it sticks better or is lower to the ground or something.

AO

1. Yes it will affect fuel economy adversely - but I doubt you'll notice it too much.

2. at 60MPH your new tires will read 57.7 MPH

3. I believe your assumption is correct.

mark kibort

im commuting to the race track driving 80mph most of the way, and still get 20-22mpg.
I think there are other factors that far out weigh the rolling friction difference at 60mph.
mk

hacker-pschorr

Depending on the year of your car, 18's were not available at all back then.

It is puzzling why the GTS was only given a 9" rear wheel. The ZR-1 Corvette rode on much wider tires (12" IIRC) so the rubber was available.

largecar379

I switched to Carrera 997 wheels on my 80 Euro:

235/35/19 front on 997 8.5X19
315x25x19 rear on 997 11X19

I lost 44 lbs from a set of 993 wheels (16 inch) and 60 series tires.

no difference in fuel mpg (as if I cared), vastly improved traction (both wet and dry), looks awesome........

Jeff928S4

- Yowzas! Much rolling involved? Garth used a small bat and pretty much used almost its fattest point. No rubbing with the 285/30/18's. I guess there is still some room to go bigger, eh?


Nice

Same here....although I am currently using Garths older Potenzas on the rear, which are pretty slick, so I'm avoiding the rain until I get some replacements

I agree...it totally updated the look of my car.

tommytomaso

Largecar....PICS...i say PIC's..my man im thinkin some Carrera S wheels would look sweet on my Freshly painted GT Silver OB.

...But i will wait till i have a few LARGE bugs worked out.

mark kibort

sometimes there is a trade off with "contact patch" and sidewall flex with the taller sidewall, smaller tire sizes. (like comparing the 225x50x16 with a 275x35x18)
I would suspect that both have near the same rolling friction, at the cost of a harsher ride with the 275. Eitherway, i bet its a rounding error at best

mk

Jeff928S4

I did forget to mention that with the larger tires, I do feel every bump in the road now.

...but I'll trade in that wee bit of bounce for the handling these tires give, anyday of the week.

LT Texan

This is very interesting to me in trying to understand what will fit.

I calc'd the total height of my 225/50-16's at 24.86 inches

using the same method, I get 24.73 inches for the 285/30-18's

Am I missing the boat?

I was worried that 265/40-18's would be too tall (calc'd at 26.35 inches)

Dan
78 5 speed


[add: looks like my calc is spot on with Jim's .207% difference above]

AO

Dan-

Your calcs seem to be correct. But I would worry more about circumference rather than height - although they are related,

The 265/48-18 would be the wrong aspect ratio. You'd want a 265/35 if available.

LT Texan

thanks Andrew