Ok, got a blow out today when the valve stem tore...so now the only tires I have on hand is a 305 30 19 for the rear. I have 235 35 19 on the front. Will that small difference screw up the awd diff?

 

To big a difference IMHO. That is 22 revolution per mile difference.

 

Your front OD is 2.78% smaller with that setup, and about 23 revolutions per mile greater. According to what seems to be the most prominent theory here (some discard it, others don't), you're just a couple of revs per mile outside of the save zone. I would put them on and not panic, and address it within several hundred miles of non-abusive driving. YMMV.
-V

 

i wouldn't run that any longer than you would a donut.

 

No, I did it for a year and had zero negative effects.

 

NOPE,,,no problem..
I have several posts about me running 20,000+ with tires .5'' different in diameter between front and rear..I drive fast ,long distances and many dyno pulls..
The Porsche Techs even said its no problem and have checked my car out..

 

with all and any due respect to johnspeed's experiential reportage which is of course instructive..

the *funny* thing about running tolerances that are outside *recommended* spec? is that there often isn't a problem,..until there is. the reality of this has to be measured against any *perceived" gain. is all i'm sayin'. front diffs don't just "fail" there is usually a period of undue stress on it that at the least contributes? if not actually causes the failures over time. that might have something to do with abs/psm cels and confusion the ecu registers when exhibiting same. notice they rarely happen when tires are well within *recommened* spec. for those for whom this hasn't happened this is a purely academic pursuit lol

so again, run whatever you like, but again i wouldn't run that setup any longer than absolutely necessary. of course, ymmv, literally. cheers.

 

The problem is you can't actually go by the published tire size...you really need to measure the diameters...these vary tire-to-tire within the same brand/size.

 

i'm with that. my point ( if ever there was one..) is that i would quickly back off any 22 per mile revolution differential btw frt/rear in the interest of keeping my front differential happy for the long haul.

 

As I have posted in many threads,,
I have measured tires on the stock rims..
The rim width is usually not the size of our cars with the tire specs.
Fact,,, A narrower rim will make a larger O. D. AND a wider rim will make a smaller OD with the same size tire.. Usually about 1/4''OD per 1/2'' rim. size.
Failures also do occur with tires near the same size which is still a rare occurrence.
So there is more then the tire size that makes that happen,As I have had many of talks at PCA outings at Porsche dealers with there Techs..
They have told me also that when our cars were current,still in warranty, they had to go along with the recommended Porsche spec size tires for any drive train claims to be honored, regardless of the failure.
Also can someone enlighten me and show me were Porsche has this spec range?? Thanks..
Just sharing my real world personal experiences...

 

VGLNTE
Opinions are like the belly buttons which we all have. Truly there are so few educated opinions on here; most speak from a very biased (what I have read in the forums) perspective. That being said a pattern will develop that does give answers over time and with enough data --> there are a good number of posts that talk about blown diffs and many can not come up with conclusive answers as to why it blew.

Look at this from a strictly engineering perspective and make your own decision with what reasons you want to run this tire combo for what ever amount of time you decide. Optimum is matching as close to rev for a rev, any deviation from that asks something to take extra stress and wear. Your car came from the factory with tires that were 0.1" spec size difference, so that is a difference of <4 revolutions per mile.

Here is a handy tire size calculator to do your own math with.
http://tire-size-conversion.com/tire-size-calculator/

 

there's plenty of information about this topic that isn't merely "experiential ( as if that might not be enough ) to caution anyone about this being "myth/legend/wives tale" for me to believe that running mismatched tires is ill advised. this is just the first i've found. do a quick search on your own

http://www.syncro.org/VCTest.html

VC Fail - Causes & Effects

A VC can fail in two basic ways: it can stop engaging altogether or it can remain constantly engaged (sometimes only at higher operating temps). VC fluid loss caused by leaking seals seems to be the most likely cause of a total failure of the VC to engage at all. When this happens, you never have 4 WD. The more insidious and costly failure occurs when the VC remains engaged when it should not be engaged. This can lead to destruction of the entire drive train, including the expensive transaxle.

There appear to be two primary causes for the VC to engage when it should not: (1) having tires that are not all the same size and wear (all 4 must be the same), and (2) age...VCs appear to have a nautural life span, at least where subjected to routine high operating temps. (One shop also claims that having a drive train that is not properly aligned also causes premature engagement, but this theory seems questionable.)

When tires of different wear or size are used, it causes the VC to engage prematurely or even constantly. It makes the VC "think" your wheels are slipping and that you need 4 WD. Premature or constant engagement of the VC, particularly at highway speeds, overheats the VC, thereby "cooking" the viscous fluid. Over time, this causes the properties of the fluid to change so that it engages the VC prematurely or even permanently, thereby stressing the other components of the drivetrain.

A typical scenario leading to a cooked VC would involve a syncro with tire sizes that vary slightly in treaddepth. The syncro is regularly driven at highway speeds for hours at a time over a period of time. Eventually, the driver notices binding in the drive train whenever she pulls off the highway into a gas station for gas. It may be subtle at first. Eventually, though, as the fluid gets cooked and ruined, the wheels seem to stiffen or bind much easier than before, and ultimately at the slightest turn of the wheel. When it gets bad, the drive train may lock up completely in the parking lot at very slow speed upon a relatively slight turn of the steering wheel. This total engagement of the drive train puts tremendous strain on the components of the drive train when the van is moving at speed under power. With continued use, the transaxle soon fails, the drive shaft and CV joints are also strained.

 

True..
Yes it is always a good idea to get a match as close as possible,,if possible BUT You have to measure for yourself on the rim because of all the variables which do change over published specs,,
Example>>>
On my NEW recent sizes,,Conti DW specs say my rear was 26.2 on their 10.5 measuring rim and fronts 25.7 on their 8.5 measuring rim...Actual size on our 11 rim was 26 and on our 8 was 25.9....
.1 difference instead of a .5 published difference..