Ok, lets just say I wanted to get a setup with a 19x9.0" front rim and a 19x10.5" rear rim.

Then let's say I wanted to run a 305/30/19 tire on the rear rim. What would be the best tire to run on the front?

Looking at the stock (base model), the total height from the ground for each wheel (((width x sidewall percentage) x2) + rim diameter) =

Stock
Front: 225/50/17 = 242
Rear: 235/50/17 = 252

The aftermarket setup I was thinking about looks like this:

Preferred setup
Front: 245/30/19 = 166
Rear: 305/30/19 = 202

Option 1
Front: 255/30/19 = 172
Rear: 305/30/19 = 202

Option 2
Front: 245/35/19 = 190.5
Rear: 305/30/19 = 202

Now the variation between the total height of the front and rear tires on the stock setup is only 10. The variation of the setup I was thinking about getting is rougly 40. Is this ok? Does it matter for any reason? I can adjust the suspension differently in the front and back with coilovers to make similar wheel well gaps, but does the different proportions matter for traction/performance/handling?

Option 1 above brings the difference closer to stock by 10 (difference becomes 30), and a 255 tire should easily fit on a 9" rim. However, option 2 above brings it back into the stock proportionality, but I have to deal with the different sidewall height percentages between the front and rear, and I don't know the effects of that (both if it will look good, and if it will handle well).

So, any thoughts........

 

Tire height matters. Both VDC and TCS are sensitive to tire height. As you mentioned, the OEM 17 inches set-up has a 10 mm difference between front and rear tires. Would I go 30 or 40 mm? Nope.

 

i would run 255/35/19's its about 0.2 inches shorter

 

jcarp: The 245/35/19 matches the OEM difference. Running a 255/35/19 brings the front and rear heights to almost matching (197.5 + 202). That is closer then the -40 and -30 setups, but it does go in the other direction, being +5.5 over stock difference. So what advantage would this have over the 245/35/19 setup?

davidv: I have a base model so no VDC or TCS. I know proportionality matters for VDC and TCS, but since I don't have either, does it matter for any other reason? (Performance/handling/traction)?

 

Why does VDC care about tire height? Tire height will be more critical for F to R weight balancing (braking and acceleration), and your speedo.

My understanding that you must maintain your tire combo to have understeer, since VDC is programmed to react to this condition?

Or maybe I am way off base?

 

I was told the speedometer isn't hooked into tire rotation, so changing tire sizes doesn't matter. I don't know where the speedo gets it's data from though. Is that true, or am I misinformed?

 

bump and why do u want to put the 305 in the rear? from my understanding if you put the 305 in the rear u would have to roll the fenders correct?

 

Yes, you are way off base. VDC/TCS/ABS all depend on wheel speed sensors mounted to each axle. Those sensors monitor the rotational speed of each wheel. If you change to a taller or shorter tire, the # of rotations at a given speed will change.

The computer expects to see the difference between the front wheels and rear wheels within a certain range. When you get outside that range, it can trigger either VDC or TCS.

BTW, there is also a G sensor (accelerometer) in the Z that comes into play with the computer and VDC.

 

The speed signal comes from the VDC/TCS/ABS control unit which gets speeds signals from each wheel sensor. If you change tire height, you affect the # of rotations per mile which will throw off the speedometer in the car. That's why it's recommended that you keep the tire heights close to stock so you don't have a lot of error in your speedo reading and you won't affect VDC or TCS.

 

I doubt it. You may have a minor difference in balance and braking with the rear in the air and the front pointed down. But the difference would be trivial.