Tire Sizes for Fikse FM-10

jhynesrockmtn · 06-29-2018, 10:24 AM

Long story but I bought my old track car back, 1983 911 SC ROW car with 3.2 transplant and other goodies. When I picked the car up and drove it home, it had a set of Cup 2 wheels on it. The Fikse FM10 wheels which are 8.5 and 9.5 *17's were stored by the guy I sold the car to and he is shipping them back to me. The tires are long gone.

I'd like to get a set of tires ordered as I have a DE I'm instructing at coming up in 3 weeks. The wheels are on their way and will be here in 10 days or so.

I can't recall what size tires I had on these wheels. I sold the car in 2012 and bought it back late last summer.

Any recommendations other than to call Tire Rack? I was hoping someone else on the board might have the same or a similar set of wheels on their air cooled 911.

KTL · 06-29-2018, 11:09 AM

8.5 and 9.5 is a really tight fit on a SC body. Are you sure those aren't overall widths, instead of the typical/standard bead seat-to-bead seat width measurement?

Anyhow, you most likely tire size for the 7.5 inch wheel would be a 225/45 and the 8.5 or 9 inch rear wheel would take a 245/40 or a 255/40

Canada Kev · 06-29-2018, 11:22 AM

Yep, that's what I have on my early Zuffenhaus wheels (which I believe are the same size) on a 3.2 Carrera body, 225/45 and 255/40. Fronts were a bit tight but had lots of room at the back.

Bill Verburg · Bill Verburg's Garage

I agree w/ Kevin, particularly in the front, 8.5" bead width is a very difficult fit, but an 8.5" overall width is a pretty typical and conservative fit.

Similarly in back a 9.5" bead width is very doable(that's what I use) but the ET has to be just right to pull it off. More likely the 9.5 is the overall for a conservative 8.5" bead.

I would want to carefully measure the wheels when they arrive

Assuming that the wheels are 7.5 & 8.5 x17 you will want to make a difficult decision, you can either maximise grip, minimise torque loss, or try for a middle ground

I used RE71 as an example of a great street & track tire w/ lots of size choices

min torque loss but still w/ good grip - 215/45 & 225/45 RE71, inertial cost = ~42+44 lb-ft

middle ground - 225/45 & 245/40 RE71, inertial cost = ~44+44 lb-ft

middle w/ more grip - 225/45 & 245/40 RE71, inertial cost = ~44+44 lb-ft

max grip - 235/40 & 255/40 RE71, inertial cost = ~48+48 lb-ft

There is an additional gearing cost for the drive tires which here is almost the same for all as they are all 24.7 to 25" OD so only ~3lb-ft difference

clutch-monkey · 08:29 PM

i have fikse wheels in that exact size on my 78 SC. well, fronts are 8.0".. rears are 9.5".
i run a 235/40 front and 275/40 rear in nitto nt01's.

IROC · IROC's Garage

I second clutch-monkey’s suggestion. I ran 245s/275s on my 8.5/9” wheels and loved them. I now have 225/255 and they look too skinny. Especially in the front...

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by clutch-monkey

i have fikse wheels in that exact size on my 78 SC. well, fronts are 8.0".. rears are 9.5".
i run a 235/40 front and 275/40 rear in nitto nt01's.

I run 8 & 9.5 also, and used to run 235/45 & 275/40 because the tires I wanted to use 245/40 & 275/35 disappeared from the market right about the time I bought the wheels

currently run 225/45 & 255/40 on the same 8 & 9.5.

The current setup is far superior due to the reduced gearing and inertial costs relative to the earlier setup. thew difference in gearing + inertial cost on the same wheels is ~21 lb-ft, even w/ a hot 3.6 that's very noticeable, grip level is only slightly reduced and never an issue.

the other issue that affects gripis the relationship between the wheel width and tire width, for max grip you want the widest wheel that the tire fits on, I try to stay w/i the spec but you can go .5" over
for a 275/40 spec is 9 to 11" most grip will come w/ am 11, you lose grip on a 9
for a 255/40 spec is 8.5 to 10, most grip is w/ a 10, 9.5 is close enough.

There is a similar relation in front

jhynesrockmtn · 06-30-2018, 05:19 AM

Thanks for the tutorial. I have a stack of records I got from the guy I originally bought the car from 10 years ago. I'll just wait until the wheels get here and get proper measurements. I've run the car at several DE's when I owned it the first time with these wheels with both "summer performance" tires and Toyo R888s on them so I know there is a combo that works.