I know this is a subject that has been beat to death with lots of posts, many getting hijacked, and lots of opinions and conjecture. I've read a bunch of that already. I know it's best to run diameters as close as possible, and I looked at rolling diameters/circumferences and revs per mile of OE N rated matched sets. I'm planning on running new rears that are 25"/834 rev/mile with existing fronts that were 25.3/821 when new and are at about 50% now. What I'm looking for is what is the maximum differential I could run safely in revs per mile or circumference without adversely affecting my front diff? TIA

 

Hi Nick

The honest answer is nobody knows for sure, and anybody telling you any different is just guessing - it could very well be an educated guess, but it's a guess non the less

I had stock size 225s and 295s on my car and replaced the rears with 315s when they'd run out
I kept the 225s on the front (but only for a short while as they'd worn out too, but the garage didn't think to mention that to me) and my car would throw-up abs and traction errors at me

I can't remember what my diameter mismatch was, but I think it was closer than what you're stating

Obviously, as soon as I put new and larger 235s on the problem solved itself

 

Spot

On.


OP- the 25.3s worn down to 50% should be, what, 5/32 less tread...so 10/32 less diameter?

Thats .3125..so a 25.3 at 50% should be equal to a new 25...or that is the question you are proposing.

Reasonable calculus, and I suspect you can safely say that you will be 'in the middle'.

I did the 225/295 to 235/315 jump...and tossed the 50% 225s, didn't want to risk it...then tossed a fine set of 235s when I went to a 265/35. (didn't want to wait on running 9" rims up front...)

A

 

therein lies the problem, sounds like. maybe as you say, your fronts were toast and the ecu couldn't handle the fresh rears. i went with the new ps2 315/30's on well scrubbed 225/40 fronts also, and mine threw a cel but beyond the momentary "abs/psm fail", they've been great. i also agonized over whether the 315/30 was within "spec" on the difference. all for naught, they stick like glue.

but new wheels and tires soon, so all bets off.

 

Using a couple of different references I think my mix will be as good as a factory approved match. Tire Rack lists N3 Bridgestone Pot. at 25.3/823 F and 24.9/837 which is a difference of 14 revs/mile. My combo assuming new rears which I have and if my fronts were new a 13 rev/mile difference. My fronts are at 50% which makes them closer yet to the rear diameter. Do you guys agree? Another confusing point is computing revs/mile does not equal the published specs. Maybe because the flat footprint and a rolling tire is not round?

 

Yep, that's my reckoning, but can't tell anybody for certain as Porsche don't have a spec in which to work to

I don't know what is 'acceptable' levels of tolerance, but I would base it on how our cars came out of the factory - If Porsche put it on there it was for a reason, we can chop and change things, but I'd definitely recommend people stay within factory tolerance. The lights on my car proved that I tried running too far from that

Johnspeed on the otherhand (as well as a couple of others on here) have run 225 on the front and 315 on the rear with no issues what so ever

Based on my experiences I would say don't do it
Based on John's experiences he'd say it's no problem

Since we are both talking from first hand experience we are both right - For our given application on our given car - This doesn't mean it will or will not work on your car though

As we can't say whether your car will be fine or not, the safest advice has to be to run as close to factory tolerance as is possible

 

agreed absolutely. but if we didn't like being close to the edge we wouldn't drive these things daily, right? toward that end, i'm thinking of running a new summer set of 'kooks.. now that ( until scrubbed..) will be squirelly.

but this time, yeah..on 8.5 x 12's..235/40/18 w 315/30/18.. proper.

 

Nick
I just checked my other thread

https://www.6speedonline.com/forums/...mps-315-a.html

My tyres (in stock size) would have had a 3 rev mismatch
If I moved to GT2 size I would have had a 2 rev mismatch

I wanted to move to 225 front and 315 rear (as I thought I still had life left in my fronts, which I later found out not to be the case, making my thread mute)

If I had done this I would have had a 7 rev mismatch - But this was assuming all tyres were new

The rears would have been taller than the fronts in this configuration, BUT, as my fronts weren't new they would have had to make more turns making the mismatch even greater

How much greater the mismatch was I don't know, but I can tell you that brand new MPS 315s on the rear with bald Conti 225s on the front were too far out to run without causing the traction lights to go off

 

Agreed, and I'm more than willing to experiment with things on my car
But if someone asks me what to do on theirs, I can't ever recommend them to gamble, especially when I know it didn't pay off on my car

BTW
I run 235s on 8.5" fronts and 315s on 11" rears

I decided I'd stick with the narrowest wheel width recommended by the tyre manufacturers for a given size as the narrower wheel will weigh less - As long as I'm within their specs I'd always go with a narrower wheel (a 11" is lighter than a 12" in the same style)

To go faster you must add lightness

 

HI,As I stated on many threads...I have ran 25.4'' fronts and 25.9'' rears for over 3 yrs, 25,000miles now,many 180+ mph runs,numerous dyno runs and 2,000 mile road trips ..AND NO problems ....I do get my car inspected by Porsche Techs.....I know other PCA guys with NO problems within a .5'' difference.......Everyone has a Theory but I did this in real world...I hope it helps you...Just sharing my info.....

 

that's the exact diamater differential i'm currently running, and your previous postings i read on this was a confidence builder also. well, that and the guys i constantly bug with dumb questions. but this issue wasn't dumb at all, it's fair and all it takes is those cels to get us wondering obsessively esp with all the talk of the viscous coupler heating up etc.

@ant..

i'm going with the 12" rear since i think that as someone pointed out upthread, the germans really do know best with zese cars. so who am i to second guess stuttgart! i want to stay with the 315/30, and on a stock rear now, i figured i'd just go with gt2 sizing all around, since that's what i'm aiming for vis a vis ride quality. but yeah, experimenting within the parameters others that have come before us and tried and have found to be "safe" is always cool. it's the only way we know what we "like" or not. eg i didn't like 19's at ALL. in fact, never again.

also, i think that porsche lists a 3%? tire size differential; front to rear is within "spec". others will say if i'm wrong.

 

what size rims did you run 265? what tire, any rubbing issue? thanks

 

This is interesting to me. I've seen bits of info about traction lights going on, but when did it happen to you? Was it near redline, hard cornering, all the time?

I.e- is there some point the car is warning you that there is a problem?

 

As I have seen and experienced with my .5'' difference....Is the light flashes on a little bit more when I hit bumps under full acceleration only and thats only with the PSM on.....Just my info..

 

agreed those sound like "normal psm working" cels. if i get squirelly or hit sections of road that would trigger the psm, i see that fairly often under fast driving conditions, but those are far different from our error/cels that are triggered by lets say, a bad maf or in fact a tire with dramatically different size that the ecu recognizes the difference and send an error message.

i get psm dash lights fairly often when i'm pushing it under certain road conditions, ( probably similar to yours? ) but never again had a cel from just changing out a tire. which did happen once, went away and no stored code.