In the UK and I think in Germany the Turbo fuchs were available as an option on late cars. They were fitted as standard on a lot of 89s. I have them on an 87 where they were the original fitment. I have fitted Turbo Fuchs with F205s and R245 without difficulty to a sport spec ride height 89 coupe and a LOW 88 coupe without any attention needed. These cars are individual enough that all have to be looked on a case by case basis and perhaps late cars were built with the possible use of Turbo fuchs in mind. I always thought that the use of a spacer with the Turbo Fuchs was more to do with increasing the track than a fitting requirement.

I'm guessing that the problems some people had with fitment of rear 9's may have more to do with the tire itself and not necessarily the construction of the car. The actual size of a tire varies a noticeable amount from manufacturer to manufacturer. A Bridgestone RE 730 245/45 may be quite different in width when compared to a BF Goodrich Comp T/A 245/45. Why this is so, I do not know. I do know that the variances in section width between "similar" tire sizes exist.

Best thing to do is look up the tire manufacturer's website to see the actual dimensions of the tire you're choosing. Might help one avoid picking a tire that's a bit wider than you might want for your application.

Here's an example of what i'm talking about:

205 vs. 225

Sorry it applies to a Saturn application, but you get the idea. All tires are not sized the same! Weird, huh?

I've posted this dozens of times, Sherwood has it on his website as does Wills tech piece. But it won't hurt to post again.

All 911 7s whether 15 or 16 have +23.3mm o/s except the 911R wheel

All 911 8s whether 15 or 16 have +10.6mm o/s

There is a 944 7x16 w/ +23.3mm o/s and big brake clearance and there is a 944 8x16 w/ +23.3mm o/s and big brake clearance.

All 930 16x9 have 15mm o/s

Any of the +23.3 mm o/s wheels(7 or 8) will fit the front of a coupe or targa w/ 205/55 tires, even the ones that are sized larger than nominal. Some fender lip work may be needed for the larger than nominal sizes or for 8s.

The 7s w/ +23.3mm o/s or the 8s w/ +10.6 mm o/s or the 9s w/ +15mm o/s will all work on the back of an SC/Carrera flared car w/o additional lip work. Many people like the look of spacers to make up for a lack of rim width. The 9's need no spacer and will fit a lowered car w/ 245/45x16 in larger than nominal sizes w/o them.

The 245/45x16 tire has several advantages over a 225/50x16 among them are smaller rolling radius & stiffer sidewall.

My 911 7s had the o/s on the back of the paddles if Todd Simpson still has them maybe he can post a pic? Why some apparantly don't have them is a mystery to me.

You do need to be aware of the dramatic physical size differences for a given nominal size. In addition there can be a 5-9# weight difference between different brands of the same nominal size.

My '76 Carrera 3.0 targa currently has 235/45x17 and 275/40 x17 w/ some massaging of the fender lips. Previously I had front 205/50x15 on 15x7, 205/55x16 on 7x16 225/50x16 on 8x16(lips were shaved for this one), and 205/55x16 on 7x16 w/ 8mm spacers. In the back i had 225/50x15 on 8x15, 225/50x16 on 8x16 and 245/45x16 on 9x16. I always tended to use the largest physical size tire for a nominal dimension. The last sets of those were BS RE-71 which when 205/55 were mounted on 7x16 rims measured 24.3" OD x 8.5" section width x 6.9" tread width. The 245/45 mounted on 9x16 measured 24.3" OD x 10.25" s/w x9.7" t/w, for comparison the 235/45x17 measure 25.5x9.4x8.5 and the 275/40 measure 25.7x11x10.

Bill V's response is spot on ! .....

However, a small point...rolling diameter ( or radius, if you will) is not much different for 225/50-16 and 245/45-16...unlike what was said by Bill.

Here's the math:
225 mm= 8.86"
50% of that = 4.43"
4.43"( x2) , plus 16" = 24.86" overall diameter.

for the 245's:
245 mm = 9.64"
45 % of that = 4.34"
4.34" ( x2) , plus 16" = 24.68" overall diameter.

I'd call these pretty much the same .....


--Wil FerchSmileWavy

Will, theoretical math is a wonderful tool but if you look at the published measured specs for any of the major brands of performance tires as follows the 245/45 is on avg .22" shorter, this difference can be easily felt(BTDT) in the acceleration characteristics of the car.

• Bridgestone SO-3 225/50x16@24.9" OD 245/45x16@24.6" OD
• Dunlop SP-900 225/50x16@24.9" OD 245/45x16@24.6" OD
• Michelin Pilot Sport 225/50x16@24.9" OD 245/45x16@24.7" OD
• Yokohama AVS Sport 225/50x16@24.8" OD 245/45x16@24.6" OD
• Michelin Sport Cup 225/50x16@25.1" OD 245/45x16@24.9" OD

The avg for the 225/50 is 24.9 w/ std deviation of ~.09797 the 245/45 is 24.68 w/ a std. deviation of ~.11661

These #s will all be subtly altered in a consistant manner by mounting on different width rims(wider rim will stretch the carcass to lower the OD, vice versa for a narrower rim) and the rolling radius will also be affected by the weight of the vehicle, but again in a consistant manner.

You are correct that the difference is small but the effect on acceleration is easily percieved and or measured.

Further the difference between tire brands has a deviance of ~.3" by judicioulsly choosing your tires you can measureably affect acceleration.

As usual Bill is the guru of knowledge.
I am running 245-45-16 on the rears and it is a perfect fit. Although I have 225 up front, which fit, it brings a new perspective to "tracking" vs the orginal 205s.

I'm not sure that that's a good thing

Bill:
I value your input and knowledge, ...but on this point I'm really confused.

You say math is a wonderful thing, but that "published measured" measurements dictate. However, you go on to show that (your basis) measurement differences of ~ .22" somehow are more valid than my ~.22" calculated difference. Why? I think we're showing the same diameter difference, within reason....using either a calculated method or published diameter method.

I still maintain that a .22" difference on a ~25" core diameter is not worth talking about. You'll see that much difference ( and more) between old tires that taken off a car...and identical new tires about to be mounted on a car ( I've actually put such tires side by side and in some cases the difference was ~1"). You're going to tell me you can feel the car pull better on the "smaller" tires? Bill...I can can telll whether the car pulls better more on the basis of a cold day ( vs hot day), than I can between new and old tires. ..... IMHO.....

--Wil Ferch
:confused:

Will, I hate this media because often the true purpose of a post is obscured, leading to confusion and misinterpretation. I try to keep posts short and to the point, this can lead to the omission of pertinent context. My only disagreement was the method for getting the OD. The nominal or sidewall labels are almost meaningless in determining the actual physical size of a tire. Even the published measured sizes can only be used as an aproximation in any discussion of the effect on gearing due to the distortion of the tire carcass by the vehicles weight and wheel width. Some manufacturers do publish rules of thumb for the carcass distortion due to wheel width and some do publish at least static rolling radius figures. These numbers will provide a better indication of the effect on gearing. However, since not all manufacturers publish these #s we are often stuck w/ the best available data for comparison purposes, which is the universally published measured sizes. In my post I collected a random sample of published measured sizes from Tirerack becase they were convienient to gather(the manufacturers sites are often more informative) and did a quick and dirty statistical analysis of the ODs. Individual comparisons can be greater or lesser to varying degrees and thus misleading, so I feel a statistical aproach(however crude) is more informative in a general sense.

I can only comment as to effects that I have seen on a dyno, 245/45 vs 225/50 where the former measured a few more hp and on my own car where I alternated between the 2 sizes for a short period of time, again the the 245/45 on 9s was noticably quicker than the 225/50 on 8s.

The same can be said of my Audi(and every other winter car I have owned). I delay the change over to snow tires which are ~.22" taller than my summer tires, due to the perceptable decrease in acceleration.

Obviously your experience my vary.:)

Bill:
Undoubtedly you will take this further response the wrong way...so let me head this off right from the start ....don't take this the wrong way, OK?


My original "counter-point" was to your statement that 245's were smaller diameter than 225's. Let's keep to this point. I say that math predicts these being the same within .22", and you so "no"...look instead to published data...yet you come up with the same 0.22" difference (+/-). Hmm...

When I point this out, your later say that tire sizes are all-over-the-board, due to maufacturing differences and how they themselves do things !

Agreed !! ......

So, how can we say that 245's are definately smaller? The manufacturer's variability will totally over-shadow the size difference you state. I think a more appropriate reply to the person asking the original question would be to say that tires like 225/60-16's ( for example) should not be used in the rear because they are too tall...or somesuch. That would be a big-enough difference to warrant a "too tall" or "too short" answer. We'd have no point of discussion on that. But that 225/50 and 245/45...taking all the variables into account including manufacturer's sizing variabilities...puts this question on a completely ( smaller) level than (say) wrongly identifying sizes in the first place... wouldn't you agree, Bill?

Taken in the greater context...note that ALL Porsche tires...from the earliest 356's... used tires with a rolling diameter of just under 25", and calling to task differnces of .22" or so, is ( in the grander scheme of things) an over-statement calling one "smaller" than the other. Even outfits like Tirerack admit that when we go to "plus 1" or "plus 2" upgrades, we strive to get the same rolling diameter, and we cannot get to a "perfect" answer. Even they would admit that small differences like under a quarter of an inch are just not worth talking about.

Again, Bill, your posts are great and of good service to this community, but this point of calling one of these two sizes different is taking the original question to a much too-fine of a point.....

To avoid possible hardship..I won't respond further...thanks for listening. Hope you understand where I'm coming from on this...

--Wil Ferch:)