nick49

I searched previous posts and now I'm more confused than ever. I also checked my owners manual and the "Essential Companion".

It seems like owners have opinions, I have read
"ok so long as they are near 25",
"diameter within 2%"
"within 3%"
"within 4%"

it's like nobody knows, but what someone has heard, sticks in their mind and that what they believe and reply to others that ask.

Adrain states:
"No data is provided anywhere regarding the impact of rolling diameter variations and front drive percentages. There is nothing in any fatory Porsche AG or ZF company documentation either." Recommends sticking to the stock sizing.

I want to run some 19" wheels, 8.5 (ET53) and 11 (ET40) that were delivered today. They were off a GT2 and came with some half worn tires that I'd like to finish off providing the clear everything. They are
325/30 ZR19 27" dia 85.75" cir 738.98 Rev/mile rear
235/35 ZR19 25.5" 82.00" 772.72 " front

Looks like to make them work I'll need possibly 7mm spacers. The front tires kiss the lower spring cup and the rears the front and rear lower plastic fender well.

What are your thoughts on these sizes as far as keeping my front diff happy? ABD, PSM, ABS, etc?

Land Jet

That's 1 1/2" difference front to rear. Better have some money put away for a new front diff. You may also get some problems from your PSM and ABS. Others will chime in.

brad@tirerack.com

Same OD 25-25.5" F/R on 994/996, .75" taller in the back on a 997 and you will be fine. That's all there really is . Call me if you need to and we can talk specifics.

adam_

Not sure what you are asking...

this issue has been discussed extensively, with calculations, rationale, and personal experiences...yet you seem to say that is hearsay but you want more inputs.

I'll keep in simple. Keep the front to rear mismatch, in terms of 'revs per mile' to the same % difference as stock sizes.

And 997 size tires are NOT acceptable

Land Jet

But if you don't believe any of the info, please put them on and drive. Then come and tell us your results as we would like to hear them.

doubleurx

+1 for Damon. He just got me 235/40/18 fr and 315/30/18 r Kumho XS. They are going on Tuesday.

LVDell

Holy crap! Run that 27" (or anything 26"+) and let us know how many minutes it took to destroy the front diff.

This has been discussed ad nauseam.

The cliff notes version is.......

The diameter isn't nearly as important as the delta in diameter front to rear. The factory diameter delivered on the car is something like 25.2" in front and 25" in rear. You could run 26" diameter without problem as long as you are the same F&R. You start getting into problems when the front is 25" and the rear is 26" or greater. Remove the front diff, run a RWD car and then you can vary more. Get too high though and then you are asking for ABS problems.

nick49

Thanks for your inputs. These wheels and tires I have were off a 997 GT2 and the front to rear circumfrencial difference is 4.40% and the measured diameter differnetial 5.56%.

You math majors can find the actual percentages, what I got was based on measureing with a tape.

The differences are greater than what I feel comfortable running with the AWD so I'll look for new tires.
Also I found out that the rears are 12" wide ET 51 which may require spacers to work well.

So I guess what I'm asking now, are what are some of the larger sizes you guys are running both in 18 and 19 sizes without issue?

x50type

nick49

re-read adam and land jet then act accordingly.

as LVDell states - This has been discussed ad nauseam. .......... search.

LVDell

Nick....the most accurate way is to find out what tire brand and size it is and then go to the manufacturer reported sizing. a 295/30/18 from 4 different tire companies can have 4 VERY different diameters.

In 18's running 225/40/18 and 295/30/18 is perfect. In the 19's most guys are running 35 and 25 series respectively. Again, check the manufacturer specs for the EXACT diameter for a given tire.

nick49

Guys, thanks!

It's not that I don't believe, just read the first 50 or so posts after doing a search and these go back to '02 or so. Didn't want to read 5k or so. Thought I'd get latest input from 10 years of discussions.

got it!

wross996tt

Here is what I mainly run with NO problems:

OZ Superleggera 19x8.5, 19x11
Advan AS 235/35 315/25

adam_

A GT2 is not AWD hence they don't worry about the impact of sizing on the AWD differential.

All differentials work, physically, to 'adapt' uneven 'revolutions' or rotational rates...hence the MOST DIRECT metric to use is the "revolutions per mile' to compare. Simply conceptualized, the difference in revs/mile will need to be 'adapted' by the diff, mile after mile.

So with stock sizes, say a 720 versus 714, the diff will rotate 6 revs every mile- at 60 MPH that is 6 times a minute. Easy. But with a delta of 740 versus 680, you are at 60 revs per minute. 10 times more heat, more wear. Just tossing out numbers here, but I find the revs/mile number translates most directly into what matters: How much more is by diff taking?

percentages of diameter dont matter, inches of diamter dont matter. The delta is revs per mile is it.

doubleurx

Revs per mile is a direct relationship to diameter - just sayin.

nick49

I understand that perfectly, and considered it before I made my first post. My original question, put another way, was at what point does the size varience become too great for differential to operate within its designed parameters?

Does anyone know what mechanical components fail when a diff goes south? I can't imagine the ring and pinion, planetary gears or bearings. I can see the viscous coupling getting stressed as the viscosity of the fluid may be too great to deal with greater than designed speed differences from front to rear.

I'm interested to know more regarding this. Not trying to be a PITA, just my nature to question what not not perfectly clear to me. Thanks!