OK so due to M/T not being able to get a production date for the ET Street R tires in the 325/35-18 size I am in a spot. I have 19 C7Z (640rwhp, aiming for 700rwhp in the coming months) that currently has a set of BC Forged wheels in 19/19 with 285/30-19 fronts and 325/35-19 rears. So my current fronts are 25.73" tall and the rears are 26.68 tall (difference of .95" with the rear being taller.

I had BC Forged make me a matching rear wheel in a 18x12. I had planned on running the 325/35-18 (26.96" tall) on these wheels. Well since M/T is saying it could be another 2-4 months before they run production on these tires and I have already been waiting for 3 months now as it is I am exploring other options. Let me tell you, the options are very slim at this point which brings me to my question. On the stock 19/20 tires the car runs a .86" difference in tire height with the rears being taller. So far the only tires I can find size wise (not sure about in stock) are the Hoosier 335/30-18 DR2 (25.91" tall) and the Toyo 335/30-18 R888r (25.91" tall). Either one of these tires will drop the tire height difference down to .18" difference in tire heights.

What is the good known working specs for differences in tire heights that wont trip the computer up? Is this going to work or am I just stuck until M/T decides to finally put the ET Street R's into production?

Ideally a 345/30-18 would possibly work, but of course no one makes a tire in a 30 series aspect ratio. I was going to look into the new Continental Extreme Contact Force tires as my current tires are the Extreme Contact Sport models, but again tire sizes are the main issue as the only tire they make that would be close in size is a 345/35-18 (27.50" tall).

Thoughts or input from the group?

TIA... Todd

 

It isn't the difference in inches that counts it is the ratio. Your current ratio 26.68/25.73=1.0369 Vs the stock ratio of 1.031. A 25.91 rear tire diameter gives a ratio of 1.007. That is probably low enough to trigger early activation of TC since the EBCM will see the rear wheel speed as being too high. Normally, TC lets you spin the rear tires a little as long as they aren't spinning at different speeds or spinning too fast. With that ratio the EBCM might tell the ECM to reduce torque or even apply the rear brakes to control apparent rear tire spin. You can get around that by reducing the diameter of the front tire if an appropriate size can be found. Based on what you are doing it sounds to me you really aren't all that concerned about the front tire other than it should hold air.

You want to avoid going into sizes over 27 inches in diameter as there isn't room in the stock wheel well to hold that size tire. They will tear your inner wheel well liners to pieces. I have run Hoosier 345/35/18s and Pirelli 325/680/18s (diameter =680/25.4=26.77 inches) in the rear of my car and they both tear up the inner wheel well. My car is at its correct ride height and rubber pick up on a soft tire can literally grab the liner and tear it. I run a 305/645/18 (diameter = 25.39) or a 305/660/18 (diameter = 25.99) in front with the 325 rear tires. I have no TC issues with either one although I rarely run these tires in any mode but Track/Sport 1, Sport 2 or Race modes. If I happen to run them on the street I just run in Tour Mode.

Bill

 

Thanks Bill, so it seems I am stuck until M/T ships me the tires. I have searched for hours so far, 18" tire sizes are really limited for anything that will keep me near the 26.5" +/- 1/4" range. There literally isnt any options out there for tires it seems.

And yes you are correct, the tires on my 19/19 setup are brand new, so until I replace them I wasnt planning on changing out my fronts. If tires get back to normal in stock options my plan is to run the 295/30-19 fronts (25.97") and 345/30-19 rears (27.15") = {1.045 vs stock 1.031 or current 1.0369} in either the Continental Extreme Contact Sports or the new Extreme Contact Force tires. As of now my car is sitting at stock heights, I dont really have any plans to lower it. I do wonder if I go in and correct the tire size if it would help or no since the issue also resides in the EBCM which I dont have access to.

Thanks for the help Bill.