So I've been running a set of 18" Rota Subzeros with Kumho Ecsta Supra 712s 225.40.18 on my 2002 WRX SpeedWagon for about a month now and I thought I'd give my thoughts on the setup.

At first I was too excited to think about the performance gains or losses. Now, I have realized that the new setup offers a ton of grip compared to the crappy RE92s and the shorter sidewall allows the car to handle MUCH better. Unfortunately, even though the subzeros are relatively light for 18" wheels, the new setup does significantly slow the car down. I noticed this today when I switched back to the stockers because of a cracked wheel. The car was definitely faster off the line and was more responsive over all, powerwise that is. I'll be interested to see what the car feels like once I've got the new setup back on.

I just wanted to let those of you looking for a new setup know what you're looking at as far as performance gains and losses when going to an 18" setup. Unless you're going with a $2000+ set of forged wheels, I would stick to the 17s if you're truly concerned with performance. You will have a bit more tire between the road and the wheel to help prevent cracked wheels(doh!!) and give slightly better ride quality (which doesn't bother me even with 18s).

It would be nice if someone could look up the weights for the stock wheels and tires and the subzero wheels and kumho 712 tires. I think the difference lies more in the fact that the majority of the weight on the 18" setup is located further away from the axle rather than the fact that it's a few pounds heavier. Just food for thought.

 

You are slowing down only from any percentage of the following three factors:

1. The tire/wheel combo weighs somewhat more than the stock combo.

2. The diameter of the 18" combo is significantly larger.

3. The 18" combo is hugely wider, say 255/35/18s vs 205/55/16s.

Any pronounced combination of the three can cause the slowdown syndrome, but it usually lies between increased weight and larger diameter.

I'd be curious for you to measure if its actually slower around a track, or just feels slower.......
-M

 

The Mirror makes valid points. But #2 can be explained a bit. If you get a 18" wheel/tire combo that is the EXACT SAME weight on a scale as the original 16" OE combo, the car will still see some decreased performance in acceleration/deceleration. This is because the bulk of the weight of the combo is moved further from the center; making it more difficult to start/stop spinning.

This is why I regularly recommend to "performance minded" customers to opt for smaller, lighter wheels. Sometimes 18s are necessary to cover bigger brakes, but you must live with acceleration/deceleration "side effects" of a larger diameter wheel.

 

BTW, weight of stock wheel - about 16.5lbs. Stock RE92 - about 21lbs.

 

Interesting. I've never heard of the "further from center" factor, but it makes sense. Thanks.
-R

 

my 2 cents of physics (simplified)
Torque = Force * Distance

larger distance increases ammount of torque necessary

 

i think centerline doesn't make a wheel with subie specific offset.

also, i weighed my kumho/rota setup at auto innovations, and it came out to 40.0 pounds versus 37.5 for the stockers according to my man dexter.

i wish there was some way of calculating the weight distribution by distance from center. or total torque/rotational inertia of wheel/tire combo