Currently have 17" standard rims. Considering 18 inch. What is widest width I can install without issues. Tire size? Finally what will it do to my speedometer reading and mileage

 

Pretty much the standard limit is 11" wide without having to roll fenders or add negative camber. That being said, you would need to find an 11" wide wheel with an offset of ~+58 or higher. You can easily run a 295 without issues, but 305 width would most likely require an offset of +62 or higher to fit. If you run the correct tire ratios it should do nothing to affect your speedo readings.

 

Thanks. I came across a set of 8.5" and 11" wheels which are from a 2005 Porsche that were never used. Owner swapped them out for more expensive wheels and they are now up for sale. Will contact him about offset
Thanks again

 

Here you go:

OEM 997 10 Spoke Factory Alloy Rims:
18" x 8.5", 40ET
18" x 11", 63ET

OEM 997 10 Spoke Factory Alloy Rims Tires:
225/40/18; diameter:25.08"
285/30/18; diameter:24.73"

As for your front tires I would remain OEM 225/40/18. As for your rear tires 305/30/18; diameter:25.20" would be the widest tire. Nonetheless, I would suggest: 295/30/18; diameter:24.96" simply because the tire size if more readily available. Also, the effect on your speedometer would be minimal: 60.0 mph = 60.2 mph. In terms of offset, 996 OEM 18" x 8" front + 18" x 10" rear offsets are 50ET front + 65ET rear. Thus, OEM 997 10 Spoke Factory Alloy Rims are within acceptable specificatons given the additonal .5" 997 front and 1.0" 997 rear. FYI, no spacers required. Good luck.

 

Tghanks for the specific details. While waiting for responses I GOOGLED the offset issue and this just confused the issue with tons of data. I assume your comments refer to Positive offset????

 

your speedometer may be off about 4mph... I know because everyone passed me in a 20mph school zone...my gauge say 20 but I'm actually going 16mph..

 

That seems unusual. Porsche did sell optional 18" wheels and I would assume the rim and aspect ratio were such that they did not need to adjust speedo for optional tires. Your stated 20% difference is 16/20 mph. Prior respondent indicated only a 0.2 MPH difference at 60 MPH. Could it be you have oversized tires from the Porsche spec??? I am confused

 

Exactly what I was thinking. With all do respect to MkC2, to achieve a 20% speedo difference the section height would be between 55 - 60mm. Completely off the OEM chart, and dangerous for that matter.

 

I think the original wheel was 17" & the previous owner bumped it to 18".....
the front tires are 225/40/18 ET 52 & the rear 285/30/18 ET 58...don't know if that makes any diff. but police scanner don't lie, it clocked me @ 16mph every time when the gauge say 20

 

nice set up

19x8.5
19x11


http://wheeldynamics.com/porsche_wheels.php

 

Your numbers are fine. You're speedo should be off by less than one mph of OEM.

Your kidding, right?

 

Whats the widest tire that can be ran on a 18X10?

 

nope, the po po setup there big scanner by the school zone & I always get clocked @ 16mph when my gauge say 20...maybe my speedometer needs to be reset or something, who knows

 

People have been known to squeeze a 315. But, I would not recommend this. 305 is pushing the envelope. And, 295 is beginning to push the envelope. 285 is the optimum choice. Yet, 275 seems to be the most compatible considering the P-car's front tire ratios. Go figure.

 

295 30 18 rear. Speedometer reads consistantly 3mph (upto about 30-40mph) - 5mph (triple digits) higher than actual speed. I run a gps based speedometer side by side.