Check My Math on Fuchs Rim/Tire Clearance

[EmeraldE]

I have a ‘73T chassis with RS flares, and I’m comparing two rear wheel/tire combinations. If I use the slightly narrower but taller 215/60 CN36 vs my current P7 225/50:

1) will I likely have less fender clearance if used to replace my current P7 225/50 tires on 8x15 Fuchs?
2) would I have approximately 25mm additional fender side clearance if I use a 7x15 Fuchs for the rear wheels, or will the 215/60 tire clearance be similar to that using the 8x15?

Specs as shown on Longstone Tyres site:

Pirelli 215/60-15 CN36 tires, 639mm diameter and 228mm width
Pirelli 225/50-15 P7 tires, 608mm diameter and 229mm width

Yes, I double checked and Longstone shows the 215/60 CN36 as only 1mm narrower than the 225/50 P7.

Based on some old Pelican threads about Fuchs wheel dimensions, offsets, and front spacing, the ‘front-spacing’ using a Fuchs 7” x15 is 73.5mm (2.9 inches), and using a Fuchs 8” is 98mm based on the 10.6mm offset version.

Thanks.

[Bill Verburg]

Originally Posted by EmeraldE

I have a ‘73T chassis with RS flares, and I’m comparing two rear wheel/tire combinations. If I use the slightly narrower but taller 215/60 CN36 vs my current P7 225/50:

1) will I likely have less fender clearance if used to replace my current P7 225/50 tires on 8x15 Fuchs?
2) would I have approximately 25mm additional fender side clearance if I use a 7x15 Fuchs for the rear wheels, or will the 215/60 tire clearance be similar to that using the 8x15?

Specs as shown on Longstone Tyres site:

Pirelli 215/60-15 CN36 tires, 639mm diameter and 228mm width
Pirelli 225/50-15 P7 tires, 608mm diameter and 229mm width

Yes, I double checked and Longstone shows the 215/60 CN36 as only 1mm narrower than the 225/50 P7.

Based on some old Pelican threads about Fuchs wheel dimensions, offsets, and front spacing, the ‘front-spacing’ using a Fuchs 7” x15 is 73.5mm (2.9 inches), and using a Fuchs 8” is 98mm based on the 10.6mm offset version.

Thanks.

Here are the full specs for what you are proposing

The tread width doesn't change but the section width does when the same ire is mounted on different width wheels

the biggest difference is the tire height, the 215 costs ~18 lb-ft. of torque due to it's extra height

[EmeraldE]

Bill, I’ve followed your comments/descriptions/data several times over the years, especially when I was doing DE in a ‘78SC ~15 years ago, and have printed out several versions of the charts that you have posted. I was using my 12+ year old printout of your charts this morning trying to figure this out myself, but your post went well beyond the info that I’ve used before. Thank you very much!

Would you mind providing a similar comparison for just the CN36 tires, 185/70-15 and 215/60-15, using either 6 & 7 inch Fuchs vs. using 7 & 8 inch Fuchs?

And regarding the torque loss of the 215, for the first time in my 43 year 911 experience I finally have enough torque in my ‘73 that I can accept that penalty, especially I no longer do track events. But thanks for noting that.

Best,

Bob

[Bill Verburg]

Originally Posted by EmeraldE

Bill, I’ve followed your comments/descriptions/data several times over the years, especially when I was doing DE in a ‘78SC ~15 years ago, and have printed out several versions of the charts that you have posted. I was using my 12+ year old printout of your charts this morning trying to figure this out myself, but your post went well beyond the info that I’ve used before. Thank you very much!

Would you mind providing a similar comparison for just the CN36 tires, 185/70-15 and 215/60-15, using either 6 & 7 inch Fuchs vs. using 7 & 8 inch Fuchs?

And regarding the torque loss of the 215, for the first time in my 43 year 911 experience I finally have enough torque in my ‘73 that I can accept that penalty, especially I no longer do track events. But thanks for noting that.

Best,

Bob

The widest wheel a 185/70 is spec'd for is 6"

I've gone .5" over and its a stretch

add ~.2" to section width per .5" of extra wheel width

1/2 on each side


the widest wheel a 215/60 is spec'd for is 7.5"

[Bill Verburg]

here's a 6 & 7 vs 7 & 8 w/ the same 185/215 tires

[EmeraldE]

That’s really helpful Bill, thanks very much for both comparison charts and your explanation.

This gives me the confidence to go ahead and order the tires. I’ll post back once I’ve mounted and tested them.

There’s a cold beer (or whatever your favorite drink is) at my expense whenever you’re in Atlanta.

Best,

Bob