I have noticed that I have lost power after having my 18" Z06 Wheels on. I went from 245/50/16 to 275/35/18's. I know that the tire height is about 1/2" taller than the stock tire. I havn't lowered it yet, planning on doing that soon. I was told that I have change the height of the car so it changed the center of the car. Could this change the power too? Any suggestions would be greatly appreciated.

Thanks
Tuff

 

The only thing it would affect would be your gearing. Here, fiddle with this for a little bit. I wouldn't think it would change it enough to be noticeable though. Why do you think it's lost power?

 

How would changing your wheels affect the power output of your engine?

How did you verify that you lost power? Dyno? Track? Assmeter?

 

Actually, after I looked at the gear ratio calculator I showed you, it doesn't even change the gearing any. They're almost identical. Now I'm stumped as to why you think you lost power. Unless of course you're going by the fact it won't spin tires as easily.

 

Bigger wheels and tires are heavier. http://www.angelfire.com/my/fastcar/suspension.html has a chart showing wheel weights.
I noticed the difference when I went to 17s.

 

What weighs more, wheel or tire?

 

An increase in 15mm is about 6% more than what you had before with the p245 tires. That means you will put 6% less torque to the ground at any given rpm.

increasing the tire size has the same effect as decreasing the rear gear size. In both cases, the tire will require fewer revolutions to reach a given distance. On the other hand, shift points will also be at higher speeds now.

You dont lose HP with bigger wheels. You lose torque that goes to the ground. You can maintain that lower torque through a higher speed before needing to shift, because redline will be reached a higher speed in each gear.

HP = torque x speed. By increasing the wheel size, you decrease torque while increasing speed, keeping the HP the same. There are several discussions of gears, HP, and torque relationships on this board.

 

its because of the weight of the new setup compared to the old...its reciprocating mass that slows you down, thats why people go to drag wheels, aluminum driveshafts, lightweight flywheels, underdrive pulleys, etc....its to reduce the weight that is getting moved

 

I had the 17/18inch rim combo back in the day...

I noticed the car seemed slower with the 18's in the rear...

I thought it was just me but I guess not...

What causes that, I havent a clue.... But dont feel bad, I felt it too...

 

Well, I haven't taken it to the dyno, but it just seems that I don't have the response that I had before I put the wheels on. Maybe it is a loss of torque, it just seems to bogg down now at launch. It is a A4, running 3.23 gears. Does anyone have any suggestions? Would be greatly appreciated. Thanks to all who have replied.

Thanks
Tuff

 

u have lost 6% torque to the rear wheels. U should notice your rpms are a little lower at any given speed/gear compared with before the wheel swap, unless the pcm has not been speedo calibrated for these wheels. In that case, you're going 6% faster than what your speedo tells u.

 

I have had the PCM reprogrammed (PCMforless), but my tranny is a 4l60. It's not electronicly tunned for the wheel size. I will have to change the gears in the tail of the transmission. Will changing the gears in the tail shaft help? Thanks again to all.

Tuff

 

going from 3.23 to 3.42 will help will bring you back to where u were before. Going even higher will net u more torque, but you will be revving higher in your overdrive gear on the highway and freeway.

every time you change a wheel or gear size, u need to recalibrate the pcm's speedo output.

 

As said above, reciprocating mass is the major culprit. Weigh a wheel/tire combo and compare it to the previous combo, then multiply it by four. You've not only added weight, but since it's reciprocating, it has greater effect on output than just adding weight elsewhere.

 

It is unlikely that the difference in rotating mass between the 18" and stock wheels would have a noticeable effect on acceleration on a car that weighs over 3500lbs.