Same tires size for front/rear

[wfbal]

I'm wondering what would be the effect of putting same tire size on front and rear axle.

I have an SC 79 with a set of Fuchs 15 x 7&8 and a set of 4 DOT racing tires almost new, in size 222/50.

I know usually we fit wider tires in the back, would any one have experienced such a set up?

[stlrj]

I'm using 6X16's (Design 90's) all around with 215 60 16's with no adverse effects. In fact, I can't tell the difference in the handling dept and I drive them hard.

Just remember, Porsche didn't start putting bigger tires in back until '78 with the SC. That means that Porsche has had decades of experience with the same size tires on all their earlier models.

Cheers,

Joe
74 w/SC flairs and 86 3.2

[wfbal]

Thanks Joe,

I will try the 225 in front as well.

[wfbal]

Well, after some research, I will answer my own question!

Same size tires front/rear on a 911 is a big no no.

It might help the understeering tendency at low speed cornering but might induce oversteer at high speed.

You (and I ) don't want that.

[no substitute]

Those nutty, whacky guys at Weissach. They actually think they know what works well on their cars. I'm joking here, as your question is perfectly plausible at least for consideration, as is your solution to same. The reason your question interested me was the experiment European Car just conducted with a 996 (I think that's the model they used). They found that by going to a slightly larger tire there were some benefits to be had, but over all the size fitted by the factory worked the best, much to their surprise. Porsche wasn't the least bit surprised and simply stated that the car's suspension, fuel components etc. are designed around the wheels and tires or something along those lines. As you stated, at low speeds no big deal, but near the limit it's a different matter. While your situation is slightly different from EC's experiment, the results were telling.

[67nin11]

what about the ST?

[Rod.911S]

My '77 2.7 has four 7" Fuchs (currently with 225/50/15 tires). According to the paperwork that came with the car, the wheels were dealer installed options when new. I have had the car less than a month, so I haven't much experience but the PO autocrossed the car with no problems.

Just for the record, the 1977 factory set-up on the 911S had four 6x15 with 185/70VR15 and the Turbo Carrera had front 7Jx16 with 205/55VR16 rear and rear 8Jx16 225/50VR16.

[dd74]

Same-sized tires (width, I assume) work fine as do wider tires in the rear than in front. Before I flared my '74, I had 15x7s and 205-50-15 on all four corners. The set up worked fine, both on the street and the track. Now, with my car flared, I use 15x8s and 225-50-15s. Again, this is fine. It's better, in fact, but to address your question, either same sized tires or wider tires in the rear work fine. I imagine the reason PAG flared the rear of their cars had ultimately to do with the increase in power of their engines, as in the case of the 210 hp Carrera in 1973.

[aigel]

Walter:

I run kumho victoracers on the track with 225 front and back on 15x7 and 15x8 fuchs. Sounds like you may have the same tires! There is nothing wrong with doing that. Ideally you'd want 245 in the rear but that is not availabe in 15" from anyone in an R compound except for Hoosiers (pricey).

You can theorize all you want, but the same size tires front and back on my SC get me around the local tracks in lap times that are comparable to anyone with 225/245 tires that has a similar car and similar driving skill. If you feel any "imbalance" you may adjust that by varying tire pressure. Nothing you'd worry about on the street, but on the track a few pounds back or forth can help. There is other ways to achieve better balance, e.g. with adjustable sway bars.

If you are uncomfortable with the difference (we are only talking 20mm), go buy a set of front 205 tires.

George

[Cookie Porsche]

You can use the same size, e.g. 225 front and rear. I would suggest a 225-45 (versus the 225-50) if available, but thats with DE's in mind.

The 225 is a 'bit' too wide for the 7J, thus is will still have slightly less straight-line surface contact than on the 8J. The added rubber will however be benefitial.

Most STANDARD 911s suffer from initial understeer. The added rubber will eliminate some of that. The front tire will not cause oversteer coming out of a corner (unless of course you don't let go of the steering wheel...!). You will also have more rubber to stop the car.

I have run 225 all round and agree with Rod and George. No problem.

Regards
Tom

[dd74]

Tom - are you truly in Africa? What part?

[Cookie Porsche]

Yep - South Africa, Johannesburg

Permanent sun, little rain, lots of racetracks and plenty Porsches - Africa is a dark place...

My version of Karen Blixen: I had a Porsche in Africa...

I visisted LA twice last year - I would be an addict if I lived there. You have more Porsche shops than Taco Bell's! Some impressive, some good and some - well, leave that.

Next time I'll bring another 20 suitcases.

Regards (from sunny SA)
Tom

[IROC]

Porsche used the same sized tires front and rear up through the '77 models, I believe, so I wouldn't call it a "no no". My '76 originally came with 6x15 Fuchs and 185/70-15s all around.

I ran 225/50-15s all around on 7x15s before I flared my car and they worked fine. In reality this works great as it dials out some of the inherent low speed understeer. As long as you're prepared for the slight tendency towards oversteer at higher speeds, you're all good. If you're not tracking the car, this is even less of an issue.

Mike

[Elombard]

Me too I run 225/50 15s all around on 15x7,8s works great.

[wfbal]

My car will see quite a bit of track time.

I will do Solo 1 racing and DE. Since most of my Solo 1 races are on slow tacks, I gess I will stick to my actual set up (225 front and rear).

DE are on a fast track (Tremblant, old formula 1 track) so I will probably use my street set up for that (6-7x16 fuchs with 205 & 225).

Thanks for the feed back guys.

[Elombard]

I run tracks with the 225 all the way around. No problem, tire pressures and adjust sway bars to fine tune - FWIW