Tyre size - cooper cobra

[Crom Cruach]

So there’s loads of info here on tyres but I can’t seem to find the definitive size I need to choose for cooper cobras for the rallye wheels. According to the receipt from 1975 (see attached photo, found in the glove box), The original tyres were GR70 (I think 15). I’m reading 225/70/15 would be close to original, 235/70/15 would work too. Does this sound correct? Not sure how to check 

 

[1970mcss]

My car has 225/70/15 on 15x7 stock rally wheels. Some have gone with 245/60/15.  Check out the tire comparison tool. 

Tire Size Comparison

 

[Canuck]

225/70/15 will be very close to the stock tire.

 

I went with 255/60/15 Coopers on the stock 15x7 rally wheels on my 71 Monte

 

 

[MC1of80]

Barn Find has 235/70r15s.

[MC-71]

Number two is the look for me. That’s what is on my Monte.

 

[Dtret]

225/70/15 on BF Goodrich. Still green lettering so they won’t turn brown.  Yet. 

[Crom Cruach]

You getting ready to fit rear glass Dennis?

they all look great. So I guess 225/70 is near to original, and 245/60 is basically wider & lower? Does anyone go 70 rear / 60 front for an angled stance? 

no clearance issues with 255 Canuck?

[Canuck]

2 hours ago, Crom Cruach said:

no clearance issues with 255 Canuck?

None at all. Lots of clearance on the stock 15x7 rallys.

Note that 255/60 has a similar sidewall height to the 225/70.

[Dtret]

2 hours ago, Crom Cruach said:

You getting ready to fit rear glass Dennis?

they all look great. So I guess 225/70 is near to original, and 245/60 is basically wider & lower? Does anyone go 70 rear / 60 front for an angled stance? 

no clearance issues with 255 Canuck?

A lot closer now than when this picture was taken. I’m waiting on a part then the glass guy will do front and rear and he is actually waiting on me. Lol. 

[sbarnette]

14 hours ago, Dtret said:

225/70/15 on BF Goodrich. Still green lettering so they won’t turn brown.  Yet. 

Good to see another picture Dennis! Haven’t seen too many updates lately.

[Dtret]

3 hours ago, sbarnette said:

Good to see another picture Dennis! Haven’t seen too many updates lately.

That’s actually on older one but still no updates. I moving further backwards than forward. So I won’t update lack of progress. Lol. But hopefully soon. I’m off work till Tuesday so let’s see what happens. 

[jft69z]

3 minutes ago, Dtret said:

 I moving further backwards than forward. So I won’t update lack of progress. Lol. 

Sounds like my story, lol. Don't be shy, share the misery with us as well, makes us feel better about our issues 🤣.

[Dtret]

1 minute ago, jft69z said:

Sounds like my story, lol. Don't be shy, share the misery with us as well, makes us feel better about our issues 🤣.

We all don’t have that much time. Besides I would wear out my phone. Lol. I like to cry alone. Lol. 

[714024SPEED]

Blue car 235-70-15

Tan car 215-70-15

Both with 15x7 rim

[PatB72]

Is anybody running 15x8 rally’s? If so can 255/60’s fir on front without rubbing issues? Thanks

 

[Canuck]

3 hours ago, PatB72 said:

Is anybody running 15x8 rally’s? If so can 255/60’s fir on front without rubbing issues? Thanks

 

I have 245/60s on Corvette 15x8 rallys and they rubbed in the rear. I had to bend the quarter panel lip a little for clearance. You can get 15x8 rims from Wheel Vintiques that have more backspacing which would allow a wider tire on a Monte.

 

I have 255/60s on 15x7 rallys on the other Monte with lots of clearance.

 

[Mikstudie]

265/60/15 rear   245/60/15 front    but the stance on my car is a bit higher on all four corners than stock..

[Mikstudie]

[Scott S.]

18 hours ago, PatB72 said:

Is anybody running 15x8 rally’s? If so can 255/60’s fir on front without rubbing issues? Thanks

 

Just on the rear. i believe that using them on the front, depending on which offset you use, might rub against the steering linkage in sharp turning. As for tire contact, the age of your front suspension will determine how restricted your turning will be. But odds are pretty good that there will be some contact with that profile of tire. 

[Mikstudie]

14 hours ago, Scott S. said:

Just on the rear. i believe that using them on the front, depending on which offset you use, might rub against the steering linkage in sharp turning. As for tire contact, the age of your front suspension will determine how restricted your turning will be. But odds are pretty good that there will be some contact with that profile of tire. 

on my car 255/60s rubbed on a hard turn, no problems with the 245/60s

[MCfan]

I am running 255/60-15s on the front but on 15x7 rims with 4.5" backspace (+12mm offset).  I have a one inch spacer in my stock (code AO) front coils that eliminates tire rubbing.

Using an 8" wide rim on the front may increase sidewall width a tiny bit but it is the inside tread edge that will rub on a tight turn so that will not change between a 7 and 8 inch rim with the same offset and 255/60 tire. 

What is absolutely critical in this application is the amount of offset your new 8" rims have.  Note that stock 15x7 rally rims (both front and rear, of course) have an offset of +.30" (+8mm).  If you want your new wider tire/wheel combo to be CENTERED in the factory mounting position, you would need your new rims to have a .3"/8mm offset also (regardless of rim width). 

If you want to push the extra width of the new combo outward, you would need to reduce the offset to zero or even negative.  If you want to push the extra width inboard (not recommended due to possible frame/suspension rubbing), you would increase the positive offset.  My front rims have a +12mm offset but a +6mm might work even better.

Some conversations only talk about back space but that is dangerous when comparing different width rims.  For example: a 15x7 rim with a +12mm offset has a 4.5" backspace while a 15x8 rim with a +12mm offset has a 5.0" backspace.

Based on my personal experience, if you want to run 255/60s on 8" wide rims on front, I suggest you consider a 4.75" backspace which is an offset of +6mm.  That will keep the new tire/wheel combo centered very close to the stock configuration and spread the extra tire tread width evenly inboard and outboard.  As mentioned before, moving the new combo further inboard with a greater positive offset or backspace will potentially cause frame/suspension rubbing issues on the front.  Been there, done that ... Good luck.

Edited January 27, 2022 by MCfan
added photos