So - I'm just about to pull the trigger on some snow tires. The tires I have picked out are Bridgestone Blizzak LM-22 245/45/19's and 235/45/18's.

Rears are 2.28% shorter than stock, (overall diameter is 27.03", vs stock 27.66) Fronts are 2.63% shorter. (26.32" vs 27.03" stock).

I assume the Traction Control sensors are looking at the difference in rotation ratios front to back, so the fact that both tires are smaller than stock would be OK (right?).

Real question - the net difference in ratios front to back would be 0.35%. Does anyone have any idea what the threshold is where the TCS would need to kick in? Seems like there must be some lee-way. Is 0.35% within that range?

Im running a 1 inch OD smaller tire just fine..

Im running a 1 inch OD smaller tire just fine..

Exactly 1 inch smaller all the way around?

I'm trying to figure out if there's a problem with running slight different sizes front and back. (Compared to the ratio of size difference front to back between the stock tires).

Based on some tire calculator sites, here's an example: at 60mph, the fronts would be turning 61.6 mph, and the rears would be turning 61.4 mph. The ECU is going to expect to see all 4 tires turning at the same speed. Is that enough to freak it out?

Part of my problem is I don't really understand how the ECU measures traction. Is it the difference in speed between the front and rear wheels? Or is the actual speed of the rear wheel measured against some internal value table in the ECU of expected speed? Seems like it would be the former.

After looking at the speedo reading examples above, it seems the back wheels would actually be spinning slightly slower than the front wheels, which would be the opposite of the condition required for the TCS to kick in. But even in that case, is that difference enough to bother the ECU and have it throw a CEL or some kind of warning light?

-1 all the way around