I've been reading every single thread on this site and others in regards to the VDC system for the coupes. There's one thing though that I still have not found the answer to; is the magic 3% difference number based off of total tire diameter, or off of the difference in revolutions per mile? I have a 245/40/18 and 275/40/18 set up picked for my 18x8.5 and 18x9.5 wheels and while the 275 keeps the speedometer at factory settings the size difference fails diameter, but does pass revolutions per mile with the specific model of tire I am looking at, as measured by Tire Rack (most online calculators that I've seen fail it on revolutions per mile, but they are using a standardized diameter, not model specific). I've considered going up to 255/40/18 on the fronts to pass the 3% in all categories, but I'm hesitant due to possible bulge (I've had these tires in 245/40/18 before, and it looked like I could fit 255 and still have a flat sidewall or just baaaaarely bulge).

I've considered backing the rears to 265/40/18 to also pass all categories, but then my speedo is ever so slightly off and that kind of bugs me (plus a bit too much rim was showing for me last time.

Overall, what's the difference measured by VDC? Revolutions per mile, or overall diameter?

If you burn out from a stop it triggers almost immediately, same concept, the vehicle detects some wheels moving faster than others.

Exceeding 3% just makes it trigger accidentally. It's not "revolutions per mile" it's "revolutions per measured interval" and it will probably be something like 100millisecond intervals for that generation of vehicle.

So as long as the math is less than 3% per interval, VDC shouldn't set off accidentally? I know that at a current 2.7% diameter difference on my winter tires (hit over 70 here a few days ago, gotta change!), VDC is actually really lenient; I've kicked the rear out for a few feet before it tries to "save" me. 245/275 is only a 3.7% per mile revolution difference with standard sizes compared to 2.7% on said winter tires, doesn't seem like a massive difference to me.

May just go 255/40/18 on my 8.5 inch rim to avoid VDC altogether, seen a few fitments where it gives just a tiny, tiny bulge (more meaty than anything).