Does anyone know I have a 02 Z28 with 16" wheels I want to upgrade to 17 or 18",but I heard if I get bigger wheels it will slow my car down does anyone know how much hp I will lose?

 

You won't lose horsepower.

Typically, when you get a bigger wheel, you get a tire with a smaller profile, so that the outer diameter of the tire stays the same. As long as this is the case, your car's acceleration should not be affected. (You may see some differences at the launch, depending on the tire's width, profile height, and traction characteristics).

From a 245/50/16 (the stock size on my '02 Z28 -- I assume yours is the same), you can use these sizes:

245/45/17
255/45/17
265/40/17
275/40/17 (this is what I would use)
285/40/17
295/35/17
305/35/17
315/35/17 (I think 315 is the widest that would fit without modifying the car)
245/40/18
255/40/18
265/35/18
275/35/18
285/35/18
295/35/18
305/30/18
315/30/18

The 245/45/17's, 275/40/17's, and 315/35/17's maintain the closest diameter when compared to your stock tire. (0.1% bigger in all three cases).

I can't say whether or not you'll be able to actually find tires in all of those sizes, and some of the sizes probably have a lot more choices on the market than others. You'll just have to look around.

All of the tire sizes I gave are +/- 2% of the current tire diameter (assuming you're on 245/50/16's like me), so at most you would see a 2% change in your car's acceleration.

Note, however, that your horsepower is unaffected! You would pull the same numbers on a dyno regardless of your tire size. It's just a mechanical advantage. A smaller tire allows you to accelerate faster, but also limits your top speed in each gear. A larger tire will have slower acceleration, but your top speed in each gear will be higher. The numbers all change together -- a 2% smaller tire will result in 2% faster acceleration, and a 2% decrease in the top speed of each gear.

A couple more things to note. A bigger rim means less sidewall height. This has several effects:
1. Less tire flex in corners (less lean in hard cornering)
2. Less tire flex in hard acceleration (it will slip easier when you accelerate hard from a stop)
3. Less tire to protect the rim from potholes and other hazards.

A bigger rim is also (usually) heavier, which means more unsprung weight and more rotating mass. Those are bad things for handling and acceleration, respectively. Look for a lightweight rim if you can afford one! Forged rims are usually the lightest and strongest, but they're very expensive.

-Jake

 

You sort of went both ways with that answer..... first saying that the larger wheels would not affect acceleration, because you can maintain the same tire diameter, but then pointing out the larger wheels are often heavier, causing "bad things for handling and acceleration". What you need to add is that a larger diameter wheel, coupled with a smaller sidewall tire, often moves the center of mass (centroid) of the combined tire/wheel package further from the axis of rotation, and that increases the HP lost in acclerating the wheel, which shows up as less rear wheel HP.

As far as attempting to quantify the actual HP loss, its probably not worth a lot of effort. One easy example to cite is the ws6.com comparison of 17x9.5 wheels with 275/40-17 tires, compared to the same wheels widened to 17x11", and carrying 315/35-17 tires. He found that rwHP was down about 3.6 HP with the heavier wheels. I think the "appearance" factor alone is enough to offset that small HP loss.....

http://www.ws6.com/wheels.htm