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I used my calculator to enter a few possible sizes in an attempt to match the diameter of your rear tire and still stay within the approval window for an 8" wide wheel.
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Suggestion
The procedure should be in the shop manual, or if you can't find it, I'll check back and see if I can post it for you..
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Suggestion
You should take a rear wheel off and check its dimensions - including offset.
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Suggestion
I'd have to check the math, but that is directionally correct.
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Suggestion
It may be worth checking the owners manual, the tire placard on the car, or even visiting a site such as Tire Rack to determine what size the car came with.
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Suggestion
You can get them closer by going to 305/30-19 at 793 revs/mile (and good with 11.5" rims) or with 295/30-19 S04s at 801 revs/mile (but recommended wheel sizes are up to 11").
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Suggestion · Original poster
Of course the previous owner may have just ignored the PSM light, like he ignored the outrageous lip on the edge of the rotors (replacing just the pads every time).
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I just got the 2004 4S Cab with 71K miles 3 days ago. Working on some 'cosmetic' type stuff (leaks, brakes, etc) when I noticed today that on deceleration (after pushing it hard) the yellow triangle will flash. It goes away after 5-15 seconds.
I need to plug in my reader but in the meantime, is this indicative of anything in particular?
996 Newbie
Go to the page in your owner's manual that shows the instrument cluster and pictures of the warning lights (maybe around page 68) and tell us what warning light you saw.
OK...so it's not a CEL that's good. PSM light can flicker on if the PSM sensors intervene in your handling - here's what's said in the manual:
The light indicates PSM intervention even
when PSM is switched off (brake regulation in
the event of one-sided wheel spin).
Go to one of the tire diameter calculators and see how close they r. Since u r a c4 they have to be pretty close, 1-2% i think? Im also pretty sure the psm works off the abs sensors so it reads excess spin and pulses the abs.
Your car probably had 18's origianlly but if the 19's fit and they r close in diameter it wont hurt the car.
The obvious thing to me, is that both front and rear are on the same size wheel (rim diameter) and both have the same aspect ratio. There is a section width difference of 50 (235 vs 285). This means the front is of a smaller diameter than the rear. I could calculate this, if it were important to me.
So let's assume Porsche intended the fronts and rears to be close in diameter. Either a larger aspect ratio would be needed in front (40?) or less in the rear (30?) - assuming the section width remains the same. It may be worth checking the owners manual, the tire placard on the car, or even visiting a site such as Tire Rack to determine what size the car came with. Then some size calculations would be in order.
I believe that these are the standard sizes (above).
So the overall circumference is what I am concerned with. As long as the ratios of the front to the back remain the same, regardless of wheel size (18 vs 19), would be my expectation?
Yes - circumference or diameter (and radius for that matter - they're all mathematically related). Interesting that your rears have gone up one inch on rim diameter and 05 in aspect ratio from stock. I would be questioning what the previous owner was thinking when he was buying tires.
So here you go:
Stock:
Front 78.8" Circumference / 804 Revs per mile
Back 78.4" Circumference / 808 Revs per mile
What I have:
Front 80.0" Circumference / 792 Revs per mile
Back 84.3" Circumference / 751 Revs per mile
The stock seem to be dead on. My setup is just over 5% difference on the revs between front and rear. I suppose that could be triggering the PSM light to flash in certain instances.
Does anyone know what the allowable difference in front to back is supposed to be?
Looks like I also need to add 5-7% to my displayed speedo
(honestly officer, I was only doing 65...sigh
I believe the mismatch may effect the awd systems and it will certainly effect the handling. I got my car at a $15k discount because the po slid off the track and cracked the motor partly because of tires. Now its a c2 and there were a few other issues i found but there r may threads about the wrong size tires causing the front ends to skip out in hard cornering. I switched out the tires and rims, he had the same size as u and put 18" hollow twists on an factory size tires. The car handles much better.
Yes - the AWD system wants a closer match between front and rear.
No. If you were to disconnect the drive to the front differential,
maybe
you'd have less to worry about.
Even then, the wheel speed sensors are going to send information to the vehicle dynamics program that will initiate action - unwanted and unwarranted in some cases. That's why you are getting the PSM light. To minimize your risk, I would suggest going up in aspect ratio on the front. Maybe a 235/45 or 225/45-19. I'd have to check the math, but that is directionally correct.
Do we know rim width? You may be able to go with a 245 section up front if the wheel is around 9" wide. Would put even more meat on the ground. Would also add to steering effort, slightly.
I used this site. Very cool because it gives you compatible sizes after you enter yours:
https://tiresize.com/calculator/
Here are my numbers:
Seeing that I don't want to downsize the rear tires, I think my preference would be to get 245 in the front, if they would fit. Seeing that my current tires are 25.5" diameter / 9.3" wide, a 26.7" diameter / 9.6" wide tire should fit:
What do you guys think?
As well, I can get Ultra High Performance All-Season tires in that size at a reasonable cost, eliminating the need to swap tires / rims in the winter.
Thoughts?
(this is exhausting)
The C4s and Turbo have a
viscous clutch
that applies torque to the front wheels whenever there is a difference in back-front wheel speed. The more the speed difference, the more the clutch applies, you will
destroy
the viscous clutch with your present set-up , let alone
ill handling
and
dangerous
..
If you plan to use this car in the winter - I would highly recommend going with the smallest diameter wheel that will clear the brakes and the narrowest rim width; within reason. Save your 19" wheels specifically for summer.
Look at a 17" wheel and tire package for winter. Physics work against you with big wide tires in slippery conditions.
Do you agree that the 245 would fit in the front, based on what I posted above?
And yes, I might consider an entire winter wheel / tire package. But seeing I just got this less than a week ago, that is at least 6 months out of my mind. Oh wait, did I say out of my mind? lol
My opinion on swapping tires is effectively covered above. You need to be sure the rim dimensions are compatible with the tire sizes you are quoting. Do you know rim width of both front and rear? They may not be the same as what came with the OE 18" package. For example, an 8" rim will fit a 235 better than a 245 section. While a 9" rim would be more accommodating a 245. You've got a 295 tire on the rear, so I will assume that is an 11" wide wheel. Getting the specs are important.
I was going to expand on the viscous diff. This is a big problem; let's hope it's not too late.
I should retract this [[[I measured with a ruler and it is right around 9". Since the existing tires are 9.3, I figured a 9.6 (for the 245) shouldn't be a problem. And the rims extend beyond the tires (outwards), if that says anything.]]]
I suppose the only real way is to have the tire removed and the thing measured?
When I got it, I drive a few miles and heard a high pitched noise coming from the front. It didn't seem like it was speed related. It went away when I turned right and it hasn't come back since. Is that what I could expect from the viscous thing?
Luckily these tires look almost brand new. Perhaps the owner just put them on with the larger wheels.
Rim width is measured on the inside of the bead. You could easily assume you have a 9" wheel if you are not aware of this. See below:
If you are not sure if you measured correctly, there should be stampings or cast-in dimensions on the inside of the wheels. You will want to be sure of this. Guessing and rushing into an uniformed purchase is not a good plan. Didn't you already question, WHO DOES THIS?
I can't answer the question about the noise. I am not knowledgeable enough on Porsche's system to offer an opinion. But it's not out of the question that the viscous diff was objecting to what it was being asked to do.
I'll have one tire removed tomorrow so I can measure.
On a positive note (maybe), I did see that the front struts were replaced with Bilstein B6 Performance units.
Before I have the tire removed, do you think the size would be visible by just removing the wheel from the car (with full access)? Or is the size generally cast inside, requiring tire removal?
Not really any need to remove a tire. Porsche wheels have the “J” contour beadseat as shown here in the center sketch.
You can get reasonably close to the dimension inside the beadseat by measuring the overall width of the outside and subtracting 2 x 11mm= 0.866”.
I used an inspection camera and all I could see on the inside of the wheels was "R46 A980". I couldn't see much else. I'll have a tire removed tomorrow as they are probably not porsche wheels.
The brand or manufacturer of the wheels don’t matter, the bead seat area Dimensions are an industry standard that both wheel and tire manufacturers adhere to, so that tires interface properly with wheels.
Just remove a wheel from the car. Look inside the wheel - you do not need to remove the tire. There will be a lot of information in there. The rim width, wheel diameter and offset should all be there. Wheel manufacturers are required to put this information on all wheels.
The important stuff is in the first few seconds of the video:
You Tube
Take a picture and post it here if you need to have it decoded.
Yes - 19 x 8 with 46 mm offset (and J is the bead profile as previously pointed out). I'll have to see if I can find a Tire & Rim Association Yearbook (it is the bible for wheel and tire engineers). The interwebz are pretty good at this stuff too.
Can you do the same at the rear? Also, please confirm you have a 295/35 tire. I saw a few inconsistencies between your numbers and what Charlie posted for the original 18" fitment.
Looks like 245 might be a good fit on an 8" wide wheel. I'll get back with a recommended aspect ratio when I know, for sure, the size of the rear tire.
This chart is what I found as far as the tires go. It does not specify the wheel width:
As I mentioned, I would like to go with the 245 / 40 / 19. It looks like it should fit fine .. if you could give me your 0.02.
Using the tool here:
https://tiresize.com/calculator/
and what I saw on tirerack, the 245 works on a 8-9.5 wheel width.
I'll leave the rears as they are.
OK - you didn't confirm your rear tire size yet. The chart you show here lists it as a
285
/35R19. I will assume 295 was a mistake.
Based on the data you have from your calculator, it matches with what I come up with as well.
You are correct, the tire size calculator does not specify wheel width. This has to be sorted out first. Since we now know the fronts are 8", we would next focus on finding a suitably sized tire to come as close as possible to matching the diameter (circumference, etc.) as the tire you are staying with. You were able to find - at Tire Rack - that 245/40R19 is approved for rim widths between 8-9.5". You are at the low end, but still approved. This is why I was questioning you on verifying rim width. You can see that a 9" wheel would be closer to the center of the approved range.
I used my calculator to enter a few possible sizes in an attempt to match the diameter of your rear tire and still stay within the approval window for an 8" wide wheel. Not knowing for sure whether you have a 285 or a 295 rear, I created two scenarios. As you can see, the yellow highlights show a reasonably good match (same as you found). Again - this assumes you have a 285/35R19 on the rear. If it is a 295, the orange highlight is the best match I could come up with.
Thanks so much for doing the work. I definitely have 285 / 35 / 19 in the back and 235 / 35 / 19 in the front:
I am going with the 245 / 40 / 19 wheels...
All kidding aside, you all are a fantastic resource.
I would have never worried about the mismatched tires had it not had been for you guys. That would have cost me $1,300 just for the part alone (unless it's already gone and I don't know it yet).
I'll try to keep up my side and post whatever useful information I get and come across.
Tom
PS - And I'll try to keep wheel and tire straight
True Dat... Of course the previous owner may have just ignored the PSM light, like he ignored the outrageous lip on the edge of the rotors (replacing just the pads every time).
How would I know if the viscous coupling was bad? Would it begin to throw more PSM alerts?
It would not throw any PSM faults. Of course an unusual noise would be an indication.
The test procedure is not all that hard, involves having 3 wheels kept from moving (brakes applied) and turning 1 front wheel hub (with wheel and brake caliper removed and brake pads blocked so they can't overextend)) and measuring the torque at slow rotation vs fast rotation by hand (torque wrench).. The procedure should be in the shop manual, or if you can't find it, I'll check back and see if I can post it for you.. (I'd have to find it also for the detailed torque specs (Alzheimer
s))
So I went with the 245 / 40 / 19 on a wheel with a 46 offset. And guess what ... they rub. SH*T!
So my other choice is to downsize the rears from 285 / 35 / 19 to a 285 / 30 / 19 (I need to confirm the wheel width is 9.5 to 10.5). This would give just over 1% difference in Revs per mile.
Is this within tolerance?
Where do they rub? If it's a diameter issue, you may want to raise ride-height. Does the car have stock suspension? Analyze the situation before taking a rash action or throwing more money at the problem.
Actually, looking back, 46 mm is a lower offset number than I typically see on these cars. I am more used to seeing numbers in the 50s. A lower number will put the rim outward more and now that you have a wider tire, it is probably rubbing the fender lip. I'll investigate the proper offset numbers, but it doesn't help the immediate problem. Any idea where those wheels came from?
You should take a rear wheel off and check its dimensions - including offset. You don't want to keep going in the wrong direction with each change you make.
These documents list stock wheel sizes and approved tires - go to the last page of the summer tire document:
https://www.porsche.com/usa/accessor.../tyreapproval/
Your application:
The fronts should be either 50 or 52 mm offset. The extra 20 mm section width should not be a problem
with the correct offset
- UNLESS - the car has been lowered.
The rears should have an offset of 65 if they are 10" wide.
I am confused. A spacer will only push the wheel out. I have no idea how much overlap there is between the tire and spring cup (if that's what it's called).
Me either, I can't tell how much overhang there is by looking at the picture, but your tire you have now is about 5 inches larger in circumference that what is factory specified..
One good "track day" would probably cure that rub?
OK. So the rear wheels are 11.5 wide. The only tire that will fit that wheel and have the same circumference as the front is 305. Uggghhhhh.
If I knew 100% that it would fit, I'd do it. But I have no idea. (And yes, while doing the brakes today I saw that there is a spacer in the back as well)
I'd be open to buying the correct 18" wheels for front and back ... if I knew I could sell these POS 19s.
Regardless of the tire problems, I think I'm over the 'like' part of my relationship with my C4S.
I love that car.....
I've had it a week and already received 5 compliments. One woman even took pictures... (Must be the driver...lol).
I've had 305's on the back of my 99 narrow body and only had a little rub that I cured by rolling the fender. I'm sure the 305's will work on the C4S phat body..With the correct offset ofcourse..
I'd like to get the correct 18", but as I said before, I'm not into the whole $3K+ for wheels and tires right now.
If I can get away with replacing the rears, I would do that.
What size were your wheels with the 305? What was the offset?
My 11.5" have a 62 offset and a spacer, which I did not measure.
Here is a possible combination:
225/40R19 - 797 revs/mile (PILOT SPORT 4S)
305/30ZR19 - 792 revs/mile
(PILOT SPORT 4S)
I'm just surfing around the Tirerack online store looking at tire combinations that are close to each other in revs/mile.
For getting the right offset, you are going to have to do some trial and error here. Given that this combination made no sense on this vehicle in the first place, there's no promises that can be made about using spacers to get you space from the inside coils/etc and keeping it off the outside fender. You might see if you can find some torn up used 305s (or whatever you go with on the rear) for free just to mock it up and size it.
I have 235 / 35 / 19 in the front with 792 Rev per mile
The only tires that will fit my 11.5" wheel are 315 / 25 / 19 @ 801 Revs per mile (good revs. I doubt they would fit)
or
305 / 30 / 19 @ 770 Revs per mile (bad revs. Should fit)
I'm not sure where you got your
792 Rev / mile
number above. All my numbers come from:
https://tiresize.com/calculator/
.
I would tell you what I have, but I think it will just confuse you even more, Since mine is a narrow body, with different width wheels and tire sizes.
If you are still trying to match your 235/35/19 front tires, the
best
rear tire match is 285/30/19 = +.8%.. a 305/30/19 would be +2.7% and not close enough.You already know your wheels fit, so there you go...
Don't use a generic calculator. Use the manufacturers specifications because 2 different tires with the same approximate size can (and do) vary.
Tirerack shows the manufacturers specs for revs-per-mile. That's where I found that data.
So basically I am screwed if I don't have a bunch of spare tires and spacers to play with.
Bigger won't fit in the front and smaller won't fit my 11.5" X 19 wheel in the back.
SH*T
The Michelin's I linked to will fit your rear tire. May not be optimal but you aren't looking for optimal by your own admission, you are just looking to get it driveable.
You can use the Michelin examples I showed you. Pricey tires, summer only. You can possibly find another tire manufacturer that can do it, but be sure to check THEIR revs/mile, not some calculated number. Tire widths are really +- 5mm or more (notice there aren't 300s, just 295s and 305s). They don't make the tire to the exact sidewall spec, the sidewall labeling is the closest width to a "standard" number that ends in a 5!
Aspect ratios are off by upwards of 2+ percent (i.e. a 35 could be 32.5-37.5, they always round them to 5% increments, not making them exactly that size in 5% increments).
What brand and model are your current front tires? Are they in good enough shape to keep them if we can help you find a close match on the rear?
I don't want to try and match the calculated revs, I'd like to see if the manufacturer lists a size for them.
If the rears are also the same tire, then WOW. The fronts are 818 revs per mile, and the rears (assuming S04s) are 775.
You can get them closer by going to 305/30-19 at 793 revs/mile (and good with 11.5" rims) or with 295/30-19 S04s at 801 revs/mile (but recommended wheel sizes are up to 11"). But you are running 285s on 11.5s and they aren't recommended either. You might even be able to do a 285/30-19 at 808 revs/mile but that's even harder for the tires to fit on the 11.5s.
The good thing about going less wide in the back is less chance to rub.
The lowest cost option is to try the 295s I mentioned. Same tire type is a good thing. Mismatching brand and model front and rear is just sloppy. Sometimes people get stuck with limited choices especially near year end when tire stock runs low (manufacturers revamp their lines in the new year). TireRack has fewer options for our cars in december vs April.
Grab an a set of the matching 295s and cross your fingers. It�s a lot closer match for revs per mile and you can look st going to 18s down the road.
I'll probably just go on the cheap with these:
https://www.tirerack.com/tires/tires...omCompare1=yes
800 Revs is a lot closer to 818 than to 755. I don't think fitment will be a problem as the spacer in there gives a lot of room all around already.
This is a source reference. Confirm exact model applicability in the original material.
