Tire Diameter questions

[Mick_D]

What would the impact of putting tires that have an outside diameter of 26.5 to 26.7 inches on a midyear 911 with 930 flares? Isn't stock diameter around 25 inches?
The reason I ask is that the only street tires I can afford to put on my 15x11 rims are 26.7 inches OD. Not sure if there's a clearance issue or not but that extra inch and a half is concerning. Thoughts?

[tharbert]

There's a lot of good info here. Mr. Verburg pretty much sums it up that you should stick to 25"od unless you have a lot of motor.

[Mick_D]

Yeah I saw that. I was asking for a little more detail is all.

[patz]

Taller tires reduce low end get up and go. Top speed is increased but on a street car who cares.

15 inch tire selection is limited so, buy what you can afford and deal with it.

Speedo will likely be off. I kinda ignore mine and go by my GPS. If the Speedo issue bothers you, send it out to be re-calibrated.

[tdw28210]

I was getting ready to figure out how to get 325/50 r15's inside my turbo flared '77, but the tire diameter is way outside what Bill recommends. Back to the drawing board.

[porsche930dude]

way too big. my 265/45/16s are 25.5 and thats pushing it

[porsche930dude]

would a drag radial be out of the question like a 275/50-15 ? I really dont know how they compare to a normal tire. They are DOT approved so cant handle too bad

[patz]

They pick up street debris like a magnet. Flats galore.

[Mick_D]

plus they throw anything they run over in the street onto your car. Rocks, nails, screws, gravel, etc. in a few months your fenders will look sandblasted.
Looks like I'm getting some cheap rims and spacers, then. 26.7 inc hes for qa 15 inch 60 profile tire. only size that fits.

[tdw28210]

All this talk about 15"'s tires being tough to find. 16's selection seems worse to me.

[patz]

Both are hard to find.

[juanbenae]

5th gear will become even less desirable...

you don't specify if the wheels are fuchs or not. if they are at 15x11 original units you might get a set of four new 16 or 17 wheels, really highend tires, AND pay a month's mortgage if sold.... is the car a turbo for real, or a look 3.0L or the like?

[Mick_D]

The rims are Gotti J55A's, 15X9 and 15X11. The car is a 911S with the all-metal 930 flares . I will eventually buy some nice aftermarkets but for now will probably try to sell the Gottis for race wheels and buy some 17X9/17X8 Euromeisters or Fox Rotas in the RSR finish and use spacers until I can afford 11 inch wide rims again..

[Quicksilver]

I'm pretty certain you would see some serious clearance issues on the front when the wheel is turned. If you put the car through its range of turning and measure from the current tire you will see some of where clearance is a problem. You will also find some clearance problems only by putting them on. When turned to full lock the tire will probably rub the tub.

In the rear you might see some contact at full compression both on the inside of the flare and on the tube side. On the tub side I would think the oil lines would be the first place to see interference. The thing to worry about if tire contacts the flare is that it will rapidly heat the metal and discolor the paint.

Also the whole car will be lifted and if you lower the car to counteract it you are making the problem worse.

----------------------------
I have a set of 205/75R15 off road truck tires on a set of cookie cutters that I put on my car from time to time. The spec says they are 27.1" in diameter. To get them to fit without serious rubbing I have to lift the front of the car about 2/3" and I had to take the fender piece that goes right behind the bellows and push it as far forward as possible and re tighten it. Plus my fenders were rolled to begin with. If I hit a big bump it will still rub the fender in a turn. All of this relates to a narrow bodied car but it gives you an idea of what you are up against.

[OldSpool87]

I found the following calculator quite helpful in a recent wheel tire swap on my 535xi wagon. There not many people modding wagons and info is a little scarce. This tool came in handy as the rolling diameters were important in AWD set up. I also used it when ordering my new wheels for my 930. I like the calculator because it expresses any changes in current set to proposed set up graphically and in MM.

Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Sorry if this has already been passed around.

[Bill Verburg]

Besides fit and appearance issues here is the effect on performance of using a tall vs short tire. The motor is a blueprinted race SC 3.0 w/ stock gearing

[VFR750]

Bill

23" short tire is pretty small. Much smaller than 24.7-25" stock range.

[Bill Verburg]

Originally Posted by VFR750

Bill

23" short tire is pretty small. Much smaller than 24.7-25" stock range.

23" is the od of a 275/35 x15 Hoosier R6, it's a common fitment on race SC's, I think you can see why

[Mick_D]

Oh I wish I could run those Hoosiers! Might have to until I can afford the rims I need
I don't want a race car. I want a street car. Definitely going to have to get different rims.

[Mick_D]

Bill, in the graph, I'm assuming the drker line is the 23 inch right?